While trying to make a trading station with piglins, I've encountered an issue where crying obsidian or obsidian isn't being detected by a comparitor traveling through a hopper to a collection chest. I'ts as if it doesn't have enough signal strength to activate the comparitor.
I am unable to reproduce this, obsidian and crying obsidian work just fine in a hopper, please provide a screenshot of your game with your F3 debug screen enabled.