On version 1.12.2, I am able to setup a hopper that has items present in it and have a comparator attached that produces a signal if an item is present. In snapshot 17w48a, the comparator does not get a signal from the hopper at all. When right clicking on the comparator in the snapshot, the input signal will then be recognized, but gets stuck in that state until the items are removed and the comparator is right clicked again.

This can be reproduced by simply placing a hopper on any block and putting items into it. Attach the input side of the comparator to the hopper, and you will receive no input. Now, right click the comparator, and the input will show a signal. Right clicking any number of times in this state will show it retaining the input. Remove the items from the hopper, and the comparator will continue to show input. Right click on the comparator, and you will see the state clear.

Attached are two screenshots that show the above. One is showing input in the hopper with no input being shown on the comparator, while the other screenshot shows no input in the hopper, but input being retained by the comparator.