The bug
Comparators won't give output when near a container that got items in it without updating it.
Confirmed with:
Dropper
Dispenser
Furnace
Hopper
Chests - fed with hopper or dropper
Jukebox
Brewing Stand
Linked issues
is duplicated by
Attachments
Comments
Droppers and dispensers doesn't cause the comparator to update either.
A block update near the comparator will update its value.
Chests, trapped chests and cauldrons works fine.
Confirmed with:
Dropper
Dispenser
Furnace
Hopper
Chests - fed with hopper or dropper
Jukebox
Brewing Stand
When placing items manually in a chest, the comparator is updated only when closing.
Comparators also seem slow to update when reading minecarts with hopper or chest.
Also Confirmed:
Shulker Boxes

Guess I'll just add my two cents worth
It works with redstone torches.
Dupe of MC-120986.
Not the same, this started in the snapshots.
The issue got worse in the snapshots, but I updated my issue for 17w47b, so it does describe everything that is mentioned here.
Or should I revert the changes I did on my ticket so that the devs or more likely do notice this ticket, because it is newer and exclusively describes the changed behavior in the snapshots?
Because this is new behavior in the snapshots, I doubt it's the same cause in the code. Leaving is separate.
Also, the devs tend to prefer fixing issues from the snapshot first, so this may pop up for them then, making is fixed (a lot) earlier (which may lead them to your ticket too). 🙂
I made a duplicate ticket - has a extra tidbit of info. Comparators update their signal when you right click on them, but will retain the last input after right clicking. So if you right click the comparator with an item in the hopper, it gets an input state. Take the items out, and it will be stuck. Right click a couple times and it will reset.
MC-122884
I've had this happen as well.