I had this happen first on a P1S (a couple years ago), but now it is happening on a P1P as well. I had just accepted that I could only run 3 colors on that machine, but now it has started to happen on a P1P as well. When changing colors on a multicolor print, the toolhead goes home as usual, then goes to the filament cutter area. It goes to that corner, but does not cut the filament. Its not the cutter/blade. The issue is that it doesn’t even try to cut it. It moves the same way it would if you were loading filament from the external roll.
The weird part is that it does not happen every time. But gets worse and worse until that slot can no longer be relied on to change the color. You can coax it into cutting the filament by reaching back and depressing the cutter lever manually a few times and then pressing resume print. Sometimes that works the first time, sometimes you have to do it over and over until it ‘remembers’ that it needs to cut the filament before trying to pull it back.
It has to be a sensor somewhere. I think the problem is is the printer, not the AMS, but I could be completely wrong. I have taken the AMSs apart and cleaned everything of dust. I can generally diagnose and fix, but this has me stumped.