Hello Peter,
It looks a bit like a focuser slippage.
I meet sometimes with a similar behaviour that I fix by playing with the focuser tuning screws.
PY
For my case, it's the focuser tube that slips.
For sure it depends on the focuser model. Mine is a rather simple (cheap) Kepler "Crayford style" model.
It is equipped with two knurled screws : one is for blocking, the other plays on the slippage tendency.
PY
My focuser is the Esatto. It attaches to the back of the C11 and has its own internal motorized focuser draw tube. It can support 3x the weight I have on it, and it's solid. I can't see anyway its loose - and there's no adjustments to it. I have the original C11 minor locked down.
This only happens at the very end of the linear focus routine, but not always. Other movements of the focuser do not do this.
Last night, I could not rely on Autofocus to perform the focusing on its own. Every time I needed to refocus, I had to monitor it, and 1/2 the times it fail like the second image above. In those cases I would stop the process and restart it - and it would work. But when it did work - it allowed me to get crisp focus. I took 200 Green and Blue subs last night, to add to my previous Red subs of the Needle Galaxy. I now need some high resolution binned 1-1 LUMs. Here's the result so far with just the RGBs.
I just wish it was constant.