If I have a pallet with 600+75 hay on it on my main area, then switch to another area on the same save file (e.g. another colony, camp), that pallet will now have 675+0 hay on it. I haven't tested how far this goes but I have 1768+0 hay on one pallet.

I have quite a few other mods (including Set-Up Camp, Deep Storage) but none that change stack size.

ETA: does not happen when the game is paused while switching areas