FYI: Earlier today this was not happening but for some reason I can now recreate this bug all the time.

Switch out a backpack into your backpack slot (not a storage bin but a leather or grass backpack)

The backpack is empty

equip a pouch (Hugaleti Grass Pouch)

Start putting items into the pouch (not the backpack which is still empty)

At some point (Item 5 or 6) the items will start to disappear after they are moved to the pouch.

Inversely, if your pouch is empty and you start to fill up a storage bin equipped as a backpack at about the halfway mark items when moved/picked up to the bin will go poof.

It is not a big deal since nothing was lost that can not be replaced easily. Just wanted to let someone know.