Newbie, one session just went wacky - any idea why?

OK, new to astrophotography, new to Astro Pixel Processor (running trial version).  First night out with a new setup I shot four different sessions of the Lagoon area and was reasonably pleased with three.  The third - the longest and one I had the most hope for, is a mess of doubled stars.  Sort of.

I am running with all defaults generally, and realize a LOT more effort is needed to understand all the options.  But I wonder if someone might give me a clue where to look for this particular issue.  I had 61 x 30second images at 400mm.  I excluded about a dozen my first try, and 28 the second time, based on the quality score.  I had dark and bias, not flat (going to take some today). They are taken on a Sony Alpha A7Riv in raw and the raw fed to APP. The seeing is good but the light pollution horrible (maps estimate it at 6). 

Apparently (though I do not recall) I repositioned the scope a bit during the imaging, about in the middle.  The resulting accidental mosaic looked like this before any further work: 

I mention it just in case -- I had another session that occured when the scope had to flip that looked the same, and came out fine. I cropped to just that area of overlap. 

Here's the issue.  In the middle bottom (about 80% down from the top) the stars look nice and round.  This is a 200% enlarged drop, so very high magnification of very small pixels.  It's also good all the way to the left edge at that distance from the top.  But...

Here's the right side at the same horizontal pane.   It's a mess of doubling (and different colors, interestingly). 

Interestingly about the same distance down from the top as I was from the bottom before they are aligned on the right side, but similarly doubled on the left side.

It's like there is a diagonal across, running from about 80% down on the left, to 20% down on the right, that is correct -- except in the center (where you might think they cross) it is also doubled though not as badly.

It looks like an alignment problem in stacking, but I am lost as to where to look.  Note this is NOT the one where I had to flip the mount as it crossed straight up, I do not recall why I repositioned it.  I think this is when I changed batteries in the camera; it fits roughly with the time frame, and maybe that caused a bit of a shift.

I'm not asking for a magical solution, but hoping for a pointer where I might look; what kind of issue could cause this, and is it likely unrepairable, or is there a path I need to find? 

Linwood