FITS viewer performs quite a lot of things when displaying an image.
The 5D mk4 is 10 bit, so I guess the bit depth is the issue.

I had the same issues you had, but only in the FITSViewer window. But the preview in the focus tab works works flawlessly even in 16bit RAW
So you should be able to focus from there without changing to 8bit RAW.

I like to monitor my subs while they are acquired. As a workaround I simply open fits files from the file browser.
Beware it will open a second instance of KStars, so you might need to extend your swap an zram partitions

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