This issue is more discernible when you go mining, because as everyone know, the best way to avoid having an inventory cluttered by cobblestone is filling the whole pouch with just one cobblestone block in each slot.
Now comes the problem, sometimes the cobblestone (in this example) just won't go to the pouch, it goes directly to the player's inventory. It's not a big issue, but I found worthy reporting. Basically, every time you break a stone block it turns into cobblestone and goes to your pouch, that's what should happen, but sometimes one or two blocks manage to sneak into your inventory instead of stacking with other cobblestone blocks in the pouch.
I've seen this enough to clarify a bit more. If an item stack on the ground fills a slot in a pouch, the rest of the items go into the inventory of the next spot in the pouch. This happens regardless of the item existing in the next spot or not.
Direranger50 wrote: ... If an item stack on the ground fills a slot in a pouch, the rest of the items go into the inventory ...
Yes, I noticed that too, over a long time, since pouches were included. I didn't report it, because I though it's only a minor issue.
It happens with any type of blocks. It completes a stack in the pouch, the excess goes into the player inventory, despite of there's enough room in the pouch. If you drop the items from the inventory, and pick them up again, they are correctly stored away in the pouch.
This isn't actually a glitch, it's intended behaviour. I thought that people might forget about and lose track of items if things went into a pouch containing any matching item rather than just filling up stacks. But if lots of people think it should be changed, it would be very straightforward to do so.