The bug

Hoppers are able to pick up item entities inside them, even when they have a container above (see also MCPE-80555).
However, when there are currently items flowing through the hopper from a container above, the item entities won't be picked up causing them to despawn eventually.

Reproduction steps

See Minecraft1.16.4-Singleplayer2020-11-2917-51-57.mp4.

1. Build two hoppers above each other
2. Let item entities slide on ice into the lower hopper
   The item entities are picked up
3. Now put items into the top hopper and repeat step 2
   The item entities are not picked up, the hopper prefers transferring items from above