Game and Plater version
Which is the game version and Plater version you're using?
Plater-v539-Retail - DF v398 - 10.0.2

The Quest Icon is not showing next to mobs needed to complete a quest. The option is active in General Settings.
Nameplate still change color as expected.

Thank you

This indicator is, as the tooltip suggest, for quest bosses only. Those are identified via wow game APIs.
For regular quest progress / icons, get a mod from wago.io