Okay, good to know the boolits aren't being sized down. I had that experience in my .303's. The brass is fairly thick and hadn't been annealed and I was full length sizing in RCBS dies that size down for optimistically small bullets of 0.311". My rifles want 0.315" boolits so pushing then into tight and stiff necks was actually sizing them down. I went to a Lee collet die with mandrel to leave case neck insides at 0.313" instead of 0.310" and annealed my brass. Made a huge difference in accuracy.
I'd guess that blow by occurs in the chamber as the boolit is leaving the brass.
The pressure is swelling the brass and I bet the neck has expanded before the boolit leaves so gas can start leaking past the boolit. If the throat is tight and short leade then it should seal but if a bit of clearance is there then gas must leak by the boolit before is seals in the bore.
I don't think you could get melted lead on the case mouth otherwise. I know it surprised me when it happened with my .45-70... but only with that IMR4227 load. I have to think that enough gas blew by to take the sharp edge of the boolit base and leave that on the case mouth. Maybe someone else has a better idea.
Now since your boolit is gas checked and powder coated it is a little more of a mystery because the lead couldn't come off the bottom driving band as it is copper or aluminum check.
Have you recovered any boolits to check them for gas cutting?