Sooo...IDescribe's comments above about the primer seating being the culprit here prompted me to go back to the current loaded rounds and the brass that I currently have sprayed and ready to load. First, there is no detectable dried One Shot visible in the case bottom, and the flash holes look 100% clean.
Second, turns out, there are more than a few loaded rounds where the primer IS INDEED slightly elevated above the case bottom. And it's SO SLIGHT that visually and by feel you cannot see it, but when I put a parallel bar across the case base, it rocks ever so slightly on the primer. Other rounds do not do this.
I then took my primer pocket gauge to a bunch of prepped brass. I found that, despite all these going through the swage station on the 1050, there are a fair number where the primer pocket gauge refuses to sit flush. It's not a crimp, it's further into the pocket where it stops. These have about 1/2 to 1/3 of the gauge sticking out. I ran these same cases through the swage station long ago, but then I did it now AGAIN, and the result was the same: the primer pocket gauge will not seat into the pocket. Even more annoying, these same brass cases WILL ACCEPT accept priming fine on the 1050, and look normal except for above. This finding makes me no doubt the value of this GO/NO GO primer pocket gauge.....it's a Ballistic Tools Swage Gauge in 9mm.
Most the brass causing this problem (both loaded rounds and prepped brass that failed the gauge then I primed to check it) is clearly some more range pickups....S&B with a 16 stamp on them. But a few are WIN and a few are misc headstamps. None have the military cross-in-circle...
Yes, the swager is set nice and properly deep, using a half-cut case to confirm the swaging tip (anvil?) projects all the way that it can into the pocket. The bass of the swage rod wage tip abuts the base of the case, and when I set it any deeper it's obviously distorting the base of the case and wicked hard to turn any more. So based on that, it's set deep enough. I find it hard to believe that my swaging rod is wore out....I have almost no military brass I process, its there more for backup safety if one sneaks in. In reality its hard to imagine this swaging tip getting worn down by the relatively rare crimped primers I put through this press. I have >100k rounds on this press of 9mm....and this problem just popped up in the past couple month...
But should the tip of the swage rod fill the pocket? If it is supposed to, then my swage rod tip is wore out, because it doesn't go all the way up into the very apex of the pocket...even when the base of the swaging tip is abutting the case bottom.
Thanks again to IDescribe for his insight...
My next step is to load only brass that passes the primer pocket gauge test easily, and see if any manifest this problem. Then load a few that are gauge problems and see if I can re-create the problem consistently.
C