Missing items from Hopper Minecart

When Hopper Minecarts are actively picking up items from an above container while you place items in it, some of your items will disappear.

To set it up:

1. Place a Hopper Minecart with a chest above it
2. Place a stack of items in the chest
3. Now place another stack of items directly in 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.