What is the problem?
If you place a hopper minecart on a container, such as a barrel, the items inside the minecart will not be transferred to the barrel.
Regardless of the container, the minecart will suck up items on top, but as just mentioned, these are not passed down.
The only exception is if there is a normal hopper under the minecart hopper.
Why is it so important?
Many redstone constructions work not!
Even some command systems are not functional.
Really a game bug?
I'm fairly certain it's a game bug.
This error occurs regardless of the device (whether computer or mobile phone) and regardless of the world.
Pretty sure that is works as intended, you need a regular hopper below to take items out.