When items are moving through hoppers, the comparator does not give a signal - breaking systems like hopper minecart unloaders, for example. I could only reproduce this when items were moving quickly through a hopper to another storage block - when a hopper has stationery items in it, the comparator works as expected.

What I expected to happen was...

The comparator would create a redstone signal as an item moved through the hopper.

What actually happened was...

The comparator did not create a signal and stayed off.

Steps to reproduce....

1. Place a chest and a hopper on top of it, before placing a chest on top of that hopper.
2. Set up a comparator leading out of that hopper into a redstone lamp, for example.
3. Place items in the top chest and watch as they travel down to the bottom one without illuminating the comparator.

The video attached was recorded in 1.14.2 Pre-Release 4. Further information about the environment the test was conducted in can be seen in the screenshot.