Could it be, the Grease is transported over the Filament to the Nozzle and support the buildup of the brown gunk? Because it is not normal, that there is “brown gunk” buildup at all.
I had a nozzle clog on A1 with brown impurity that was quite fluid, so it creep between Nozzle and heating surface. I ended up to blame to much grease in the tangle Sensor Area that was applied during mounting in the bambu factory, as the A1 was really greasy and my A1 mini (no similar clog up to now) was way less greasy in this area. The Support denied this as a cause and i printed with a new nozzle after cleaning most of the grease and the heating unit afterwards.
I am unsure, if my theory that the grease was the main cause of the clog is wrong or if it could be correct.