I think I would pull the auger and look for dust in the bottom of the auger channel. The auger is somewhat of a loose fit, and dust would naturally fall and want to accumulate on the bottom of the auger tube. It might be acting as a
AMZN sawdust tray if there was enough and it was to become lit and smoldering, leading a path back to the pellet hopper. The pellets would be pushed out by the auger, but the dust would naturally fall out as the pellets are agitated by the auger feed.
I noticed a fair amount of pellet dust in a bag of
Pit Boss Hickory I loaded into my new Copperhead 5 the other day. I even thought about making a strainer to get the dust off in the future. I may have to explore this thought further.
One thing I noticed that is different in your manual vs my Copperhead 5 manual is the shutdown procedure. LG900 says run the temp up high (400+) for 5 to 15 minutes to burn off the grates and then hit "STOP" on the display and the controller will stop the feed of the pellets, let the pellets in the burn pot burn out, and then run the ran until the temp reaches 130* and shut everything down.
The Copperhead manuals says to run the unit at 400+ for 5 minutes then 200 for 5 minutes to burn the pellets in the burn pot, and then push the power button (off). You might try the manual shutdown method of 400 and dropping to 200 and then off after 5 minutes at 200. I think they have been shipping the upgraded controller like you have with some of the controller replacements for the
Pit Boss models lately.
EDIT----------------------
The manual talks about checking for sawdust in the hopper and auger as regular maintenance, so I suspect sawdust from the pellets is going to be at the root of a lot of these hopper back burns. It even talks about vacuuming out with a shop vac.