I'm just wonderin' aloud here.....is it possible that the nose punch for the seating die is not a proper fit to the bulet and is allowing the bullet to seat somewhat off center, or canted?
The simple test for that (if you got the tool) is a bullet runout guage.
I see this from time to time in every chambering I load for 45-70 included. Without question, none of my nose punches are correct for the bullet. Being a plinker (albiet picky at times) I've just never thought a whole lot about it. What I do when seating couple of cast bullets is, "bump" them against the seating die, (push ever so lightly is more like it) rotate the case and bump again, and rotate once more before completing the seating. I did have trouble with the sample of 31141"s that DJ sent wanting to stick on the seating punch and there was a noticable mark on the nose of the bullet just ahead of the bore rider to nose radius. Funney part is that the Hornady 150 SST's I bought, a very pointed with plastic tip design, seat perfectly and with no obvious marking or deformation of the the bullet nose itself. Factory Lee 30-30 dies, and their "O" single stage press. Go figure. If anything I would expect the Hornady bullet to at least be marked, but no spire point bullet I have seated so far shows marks or damage from this die set.