Hoppers lose items when they are created by command blocks.

Using commands, follow this scenario:

1- Place an armor stand.
2- Place a chest at the bottom part of that armor stand.
3- Place a hopper at the upper part of that armor stand.

Try dropping items on top of the hopper. You'll see they'll disappear. When you check the hopper, you'll see nothing. When you check the chest, you'll see nothing. If you check the nbt data of the armor stand, you'll see nothing. The items you dropped are gone forever!

Expected behavior: Items dropped by you should go into the chest. Or into the armor stand. It just shouldn't disappear

If you kill the armor stand, hopper will work as it should.