I had two stacks of ammo go negative for me in two separate version of the game. 2053 and 2029. While the ammo was negative I could keep reloading from it. I do not use tiles.
I had UPS go -1 (from 0) when I modded it with rechargable around version 2000.
A bit more information about the circumstances would be useful. Have they spawned with a negative count, was it the result of unloading a tool/weapon, picking up / dropping?
Sorry, I’m not sure. I didn’t notice it till I was like, “I should have run through that stack of ammo by now.”