Where did the mirror test come from? I know it has been use as a way of testing draft for some time.
I personally use the information from the manufacturer. Wait 15 minutes after the burner turns on and test with a match. If the match has a steady flame as it gets drawn into the opening, it is a proper draft (hard to fight that in court).
So the plastic overheating from back drafting during the first 15 minutes from when the burners turn on can be a possibility. Indirectly, the manufacturer does considered that acceptable.
Is that flue supported properly?