Can't shift down from 50T with clutch on, but can shift down with clutch off, why? Can't shift down from 50T with clutch on, but can shift down with clutch off, why?

I'm using sunrace 11-50 and XT RD-M8000 (SGS), finished setting up everything for absolutely perfect shifting, then realised the clutch was off. Turned it on, shifting is still excellent (but with the usual clutch effect) except it will not shift out of 50T. The shifter clicks, the RD stays put, same thing if I do multiple shifts down. I can however backpedal for a while after one downshift to drop the chain into the 42T, the RD then moves, after which I can pedal forward and carry on shifting down perfectly.

This is not a derailleur setup issue, not only have I got everything setup by the book, but I've played around with 4mm to 12mm gaps between the 50T and top pulley with B-screw with no luck.

My calculations (full formula including 2 links for derailleur, not rough estimate) showed that 113.4 (=114) links on the chain should be enough, I installed with 117 links initially, felt that was too much chain, removed 2 links and am using 115 now. Doubt it's chain length, can't see why everything would be fine with clutch off if it's chain length, but worth considering.

RD is just out of the box, so I doubt it's the Nm on the clutch, but going to check that out soon.

It's not a big issue, since there are only 2 to 5 (depending on conditions) places where I need/want the 50T and all of them are during the (mostly) climbing half of my local rides, so I can ride up with clutch off and ride down with it on. 

Above all I'm curious why extra chain tension from clutch could cause this; even if the Clutch tension is too high and that is the reason, why would this happen? After all, it shifts even under quite a lot of load with clutch off (I avoid this bad shifting practice where possible, I was just testing!)
The RD-8000 is specified for a maximum tooth size of 46 (1x11-46).  Likely you are just exceeding its design criteria and reaping the rewards of DIY mix and match.  

The clutch does not add tension. It add friction in one direction to resist a forward moving cage, but allows the spring to pull the cage back freely.  As the chain drops off a cog onto a smaller one, it has to briefly lift and this causes the cage to move forward against the clutch. The clutch is designed to slow this action - possibly in you setup just enough to keep the chain on the large cog and prevent it dropping on to the smaller cog. A longer chain will make little difference, as the clutch is friction based, so spring tension changes probably won't change the behavior. 

Chainline is probably all you have left to play with. (You don't mention the bike, presume its 1x now, was it always 1x). Moving the chain ring out a millimeter or two might make a difference - or it might not.