It's not a bug per se, but now that coffers can be spent more often (I think previously, it was just at the beginning of the buy phase?), there can be a situation where the UI prompts you do something that requires a certain amount of coins, and you don't have that many coins but you do have that many coffers.
For example, let's say I have 6 coins and 2 coffers, and a Wine Merchant on my tavern mat. I buy a gold for 6 coins so now I have 0 coins and 2 coffers. The "discard Wine Merchant from tavern mat" prompt comes up. I click it, so a coffers prompt comes up. If I choose "continue," without paying extra coffers, the turn ends and the Wine Merchant is not discarded (which is correct, since there weren't 2 coins left over).
Another example is if I buy a Doctor, using up all of my coins, but I have any number of coffers. The UI will ask if I want to overpay, so I choose overpay, then it prompts me to spend coffers. If I choose "continue" without paying extra coffers, then the turn ends.
I guess it's just weird where the UI can prompt me to do something, but then the coffers prompt comes up after, when I needed those coffers spent to do the thing in the first prompt. I can spend the coffers and do the thing, so technically everything works correctly, but I think it can be misleading and/or confusing.