I just go a similar problem with my slx mech.
I had 2 rides on this bike before,it was not a problem once I put the B screw in all the way.
I just built it with the mech off the old bike
But this ride,I heard a noise in 40tooth gear, and the jockey wheel was too close, so I changed up to 36, it was ok.
Ten minutes later, same problem in the 36, and soon in 3rd gear too
Any ideas?
The b screw is still in all the way and hits the stop
The other thing I noticed was that the mech didn’t spring back into a higher gear, the cable became loose
Could it be a clutch issue?
I have no idea how the clutch is adjusted