The give away is cracking the line at the master relieves the pressure. Something is keeping the master from relieving the pressure. Loosen the master bolts as much as possible, confirm that the pedal IS NOT against the stop or switch. Pump the brake and see if the wheel cyl is still not releasing.

If the cylinder still won't retract without reliving pressure the master is junk.

If the cylinder does retract the problem is pushrod length.

There is no mystery here, these are the only possibilities.

As for why the rf shows symptoms vs lf try swapping return springs side to side. I bet its just imbalanced springs. Maybe wear grooves in the backing plate but these are easy to see