Lesson learned. Filament frictin and MMU3. Be aware when using rough, soft or flexible filaments.
Lesson Leaned on high friction filaments.
I now double check a filaments friction potential before use in the MMU3. It is not a problem with the MMU3, but with friction in the long tubes needed for a buffer and dry box. Most PLA/PETG filaments I use there is no problem. A rough, soft or flexible filament add large amounts of friction to the system and may cause problems.
I ran into one high rated brand of PLA and color combination that had a "rougher" outer surface. With only 1 color prints, it kept sticking. Going from drybox, buffer and to MMU3. The nextruder on the MK4 would grind a flat spot and fail to feed and kept going, failed print. I imagine it would be worse on the MK3x series. Various fix attempts proved there was just too much friction causing problems.
I switched to another filament with the usual smoother surface and the problems went away.
I have had no problems running ASA ( not multi color) through the workings of the MMU. Many types of filament other than PLA/PETG will feed through the MMUx just fine. I slice as non MMU and assign the slot when print starts. This method simplifies running for me.
For rough, soft or flexible materials I would recommend not running them through the MMUx tubing. Just run these as single spools bypassing the MMU system as others have outlined for TPU.
My more than you may want to read MK4/MMU3notes.
RE: Lesson learned. Filament frictin and MMU3. Be aware when using rough, soft or flexible filaments.
I also noticed this with the ruffer filaments. Specifically the sunlu apla I think it was.
I however added the (un)original drybox, and auto rewinding spools and eliminated the buffer altogether. This allows me to shorten the tube lengths to the mmu and I made sure all tubes had an inner diameter of 2.5.
No issues since. TPU is a different beast though. I bypass the mmu with that.