Items get deleted or duplicate in bubble elevator

I'm making a storage silo in a single player creative world to build on my realm. For item deposit, I drop the items into a bubble elevator, which, at the top, goes into a water flow which leads the items into hoppers going into chests which then make their way to the bottom again if there is space in the bottom chests. However, when i drop a bunch of items at the same time, some of the items completely vanish. For example: I held my drop key down on a stack of diamonds, and when the items were depleted in my inventory, the chests held a total of 16 diamonds (including the chests at the top and in between). So I thought, maybe the items got stuck somewhere? Maybe the slabs above glitched the items? But no, they completely vanish. I tried to use the tp command as a test to see if any of the items had become stuck somewhere, but to no success ( /execute at @s run tp @e[type=item] @s ). As for the duplicate part, some items (like planks, slabs and stairs) seem to increase or decrease in number when you toss them in.

So the items completely vanish, but I have no clue as to why, and I'm supposing it is a bug.