Printing a bunch of PETG with PLA support interface, I finally got the dreaded heat creep on the aux side. The PTFE tube was not seated all the way for the aux extruder, so the filament made a little loop in the void. The soft filament also got stuck in the cutter.
I have it all back together, with fresh PTFE tube for the aux extruder. FWIW I bypassed the buffer a while ago (aux spool mounted externally on the left), and also installed 45-degree aux fan duct diverters to prevent warping.
I wonder how to avoid this issue moving forward! I will print with the lid propped up to prevent chamber heat buildup, but wonder what folks have done to prevent this problem.