I just mounted my MMU2 over the weekend and found the filament flow was not smooth enough. I noticed the s-holder-a was not aligned with s-holder-b and that was the source of some sounds while the filament is pulled to the extruder.
I decided to create a new s-holder-X thing, but finally applied the KISS principle (Keep It Simple and Stupid) and created this replacement of s-holder-a part.
You can access and download the piece from here: https://a360.co/2TN25Q1
Now the filament can flow smoothly to the extruder and the loop is created when the filament is retracted as usual.
Hope you find it useful, I just wanted to share it in case you are interested.