Hi,
I have already noticed several discussions on this.
Two days ago my Yealight bulb lights (color) stopped working from Alexa app.
No issues from Yeelight app.
Tried to reset lamps, disconnet/connect yeelight skill from alexa etc… no solution.
Temporarily solved using Samsung Smart Things app, added yeelight bulbs on it and connected Alexa with Samsung Smart Things Skill.