OISA forces inventories to adhere to TerraFirmaCraft item size limits by forcing max size of acceptable item.
When larger than allowed item is inserted into the backpack, item pops out of the inventory (expected behaviour), but also leaves the copy of the item in the backpack.

I've discussed this issue further on DoubleDoorDevelopment/OversizedItemInStorageArea#17 and not an issue with my mod.