bookbinder This sounds like you may not have budgeted the money to a category when it was received, then overspent in a category when you spent it. If it was forthe same amount and it was using a cash-based account, the overspending comes of TBB in the next month, which would offset the money that was never budgeted.

bookbinder grab a cup of tea for this one....

(sorry my comments are NOT concise; that’s a work in progress for me)

I came across a similar situation just recently.  Here is my explanation and incomplete hypothesis.  Hopefully it make sense and hopefully it describes your situation too.

I was doing some reconciliation adjustments; my husband had spent some money in cash but he didn’t know where so I used the reconciliation function to correct the ‘cash @ home’ account; it was the first time I had used this function.  And of significance - I think - ‘timing wise’, we were just entering the next month.  So consequently EARLY IN THE CURRENT MONTH, I had negative ‘cash inflow’, as summarised in the ‘inspector’ panel (right-side of budgeting screen; all categories unticked).  
For reference our household typically get all income paid on the last work day of the month.

I wonder if this was the case for you to (at the time that you were symptomatic and remain symptomatic)?  Was/is your overall cash flow NEGATIVE at the beginning of the month ‘to date’ (due to adjustments/losses) until more money comes into TBB? 

If so, continue to read on (if not ‘so long and thanks for the fish’).... Nettie

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 
Assuming your are still with me...
So the symptom was:  
In the CURRENT month after budgeting (like you) I had zero in my TBB but in LAST month I had a positive amount in the TBB!!!  ‘What the heck had happened’ I thought to myself (what steps in the last week or so had created this scenario)?  YNAB must be broke!!  I kept switching back and forth between the two months in disbelief (I only have 10 months of experience as a YNABer) - last month had money available (in TBB) but this month had NONE, yet my understanding was that ‘available money’ (if available? 🤔) would be carried forward to the next month - yet it didn’t look that way!?

Since it was 3 weeks away from receiving more income that I could fiddle with, I decide to test the waters.  I inserted ‘dummy’ income (greater than the balance of the CR held of the previous month) to see what would happen?  
Outcome: In the LAST month’s TBB credit disappeared (perfect 👍🏼 ) but so did some of the money that I credited vanished 👎🏻 so that the TBB in the CURRENT month was positive but, lower than I had expected.  Again, what the heck was happening?  I had a debt on the books that needed to be repaid it seemed!

bookbinder , I suspect you will find that when new income arrives into your budget, LAST months TBB credit will disappear. Let us know if I am correct 😉

... Experiment over, I removed the dummy money; the TBB of LAST month had it’s credit back and the current month TBB was zero.

I thought it had to be something to do with this negative transaction to balance out the ‘cash @ home” account (I think by chance I notice that the amount vanishing was the same as the NEGATIVE INFLOW summarised amount) .  But negative inflow is just like an outflow - or it is? We all have outflows (that why we YNAB 😂) but negative inflow (unlike a transaction outflow expense) was different because it was NOT coming from a defined/planned category.  Could that somehow make it different?  It seemed logical that negative cash flow could only affect the TBB fields automatically by YNAB (until you do something with it - category wise).  

So - long story short (ok it not short really 😛 ).  
• The overall TO DATE inflow for the month was a negative amount (caused by the negative cash transaction for the ‘cash @home’ adjustment). 
• I think the philosophy is that really am starting the month off in negative which is a big no-no in YNAB. It’s like I’ve budgeted money I did have (again a big no-no).  If I had known that the negative cash flow was going to hit me now, LAST month I would have held aside (in a category, not in TBB) the amount needed to cover the overage.  Since I didn’t know I didn’t allow for it (yet).  And, I don’t (yet) have income arriving to negate it automatically.  So I do need to pull it from another category and the sooner the better (that’s the YNAB way).
• Since I budgeted a month ahead I have monies flush in the current month (even though I have no income yet and in fact ‘on the ledger’ I currently have negative income because of the adjustment).  
I also note that funding months ahead can interfere with simple logic of the TBB field.
• I thought that at the very least ‘trust the process’; YNAB wants us to square it all up but it is not being specific about how it do that (it must be the magic of accounting which to the layman can be illogical until your brain is wired in the way of ‘double side accounting logic’).  
• Maybe if the ‘negative’ occurred in the middle of a month the solution would appear more logical (or YNAB would sort it out for me like the magician that it is 🪄 ); without realising it we often quickly pivot (without realising each step we do) as we instinctly use incoming cash flow or we find money (WAM’ing) remember the golden rule ‘you only budget what you have now’.  
• All other fiddling aside - and despite YNAB behaviour seeming **non-instinctual - if I was just dealing with the ‘negative inflow/income’ (different to a negative transaction) how would my logic work? The money - for the negative reconciliation adjustment - has to come from somewhere!          
[** I would expect YNAB would be prodding me along, so that I would be delivered a negative amount in TBB in CURRENT month so I would simply rob from other categories until it has lowered that negative to zero (or into a credit).  Instead it seem to not set up a negative for the current month but hold a debt shown in the ‘TBB as a credit’ 😳 which is not real; can not be used (it is not a true credit; because the current month shows TBB as zero).  In truth the TBB cr is really a negative amount (unseen) that will be be pull from new monies just as soon as they arrive].

My learned solutions for you bookbinder : 
If you want the instant solution (that YNAB neglects to alerts you to in an obvious way):
1. Lower a category - in the *CURRENT month or in ^LAST month (to free up money) to cover any negative cash inflow that is summarised (regardless of the TBB credit shown LAST month); that should remove the credit that ‘shows’ (and confuses us) for last month.  You are then all square.    

* I have flush categories in the current month (as I budget a month ahead)     

^ You can take it from money line items of LAST month but remember, they flow to the CURRENT month as part of the rolled over functionality, so if that is the option selected just flip to the CURRENT month to ensure that same line item is pumped for this CURRENT month appropriately. Sometimes it is better use the ‘first in, first out approach’ when backing out monies budgeted - take from CURRENT month then move to LAST month; it stops the need to flip back and forth between months to ‘see’ the overall impact.

 OR
2. If you want YNAB to sort it for you:
Wait for new cash to arrive (greater than your $14.04) and the credit shown in last month TBB will then vanish automatically (as the negative inflow not yet satisfied will be pulled form those incoming monies).  Once those new monies arrive (but only then) you will be ‘square’ and hopefully, there will be some left-over for you to budget.

Hope this helps to remove some of the lingering eye twitching.  And, I hope this doesn’t confuse (I did remove much of my incomplete thoughts, believe it or not!).  

Sometimes we need the answer first and, ‘the why’ will come to us later 🤞🏻 

I did say at the beginning that, my hypnosis was incomplete didn’t I 😉 

Ps - if you don’t have negative inflow, ignore all my finding, thoughts and ramblings above. 🤦🏻‍♀️