Notice recently my XT660 with the aftermarket exhaust is burning/melting some plastic that contacts the exhaust pipe before the muffler - smoke comes off it after a run of around 5 - 10 minutes. I've run with this exhaust for some time and it didn't do this before that I noticed.

It's a metal mule/Barrett type two into one exhaust.

Bike doesn't seem to be running especially hot - coolant is full and looks clean in the radiator. Running the bike with the radiator cap off, the coolant is below the inlet before start, then gradually rises as the bike runs.

Headers don't glow or appear super hot.