Easier reproduction steps in case it helps.
To set it up:
Place a Hopper Minecart with a chest above it
Place a stack of items in the chest
Now place another stack of the same item directly into the Hopper Minecart while the other items are being pulled in from above
Expected result:
When all items are finished moving into the Hopper Minecart, I would expect two stacks of items (given the example above).
Actual result:
Less than two stacks of items will remain in the Hopper Minecart. In some cases, half a stack will be missing.
Still present in 1.20.30.
To bring parity with Java, it should turn into Concrete upon touching waterlogged blocks.
Can confirm that this still happens in the latest 1.20.41