Both Nosler#6 and QuickLoad say that's not (supposed to be) an overpressure
load, so the answer must lie elsewhere. (QL says 57,500 relative to a 62,400psi MAX)
What was the real OAL, and how did you determine it?
OAL = 3.4", or 3.3
(The listed manual OAL says the latter)
Does the extracted brass show shiny ejector burnishing and/or primer/flashhole signs?
Have you pulled a cartridge (from that loading session) apart and confirmed all weights?
Q4: Is there any
chance you used RL-19 or RL-17?