It's weird it will only do it on maybe 3/5 rounds. My theory is the RL26 is temp sensitive and if you take too long to shoot it will "cook" in the chamber. The ejector marks are smears. Very very heavy bolt lift/ Velocity is low around 2900-2950 with a 147. primers look fine and pockets are still tight but only 1 firing on the brass.
The velocity seems about right for your charge. Mine is at 2975 with 55.7 grains of RL26 and the 143s are at 3020 fps with 56.1 grains, but the velocity didn't stabilize until i got to about 70 rounds through the bore.