When using addons that change the behavior of tooltips along with Questie, the feature that shows if a mob is part of a quest, doesn't work.
I've tested this with EQCompare and FlightTime, and both of them break it.

Please try v3.5 and report back. :)
https://github.com/AeroScripts/QuestieDev/releases/latest

Can you try downloading the current master branch (as opposed to the latest release) and give it another try? I haven't gotten around to testing the latest 2 pull requests in detail and haven't published a new release for that reason.

Unfortunately that seems to produce the same result as before.