Hello,
Perhaps an idea. I had the same problem with a meade Sc 8" and an atik 314L+ (So with a small field).
When I change the back focus with the camera (because I use a fliterwheel or not, ...), to focus it changes the position of the primary miror. So the focal change.
And with a 6.3 reductor, the focal is not necessary 1260.

Now, I have resolved the field of an image with astrometry.net, I noticed the focale, and I reported it in Kstars/Ekos (For me, it is 1170, not 1260). And I try to have always the same backfocus.
And I try not to touch the position of the primary miror to focus.

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