I think its the piston seals – although I’m not exactly sure why
I had the same problem with some XTR m985 brakes. Bled them at least 5 times in numerous different ways. Each time black fluid came out (not sure if this is connected) and each time inconsistent lever feel. The brakes would pull all the way back to the bars on the first stroke and then be fine on the second (if you pulled it quick enough). So this meant in order to brake you had to pull the lever twice in quick succession. A temporary fix is to take the wheel out and pump the brake till the pads sit closer to the rotor ( A process that should happen automatically as the pads wear down). This works but only till your pads wear a little more then you have to do it again.
This behavior led me to suspect the piston seals as they are responsible for returning the slave piston to the correct position and for adjusting for pad wear.
So I bought a set of SLX calipers from chain reaction and swapped over the piston seals and pistons (they are the same).
So far the brakes have been fine – adjusting for pad wear as expected
I could see nothing visibly wrong with the seals that came out but perhaps their was some invisible difference?
who knows