Fenix 6X Pro shows "Sync in progress" all the time, why?

Unless you somehow manage coincidentally to always catch it syncing (!), perhaps connect it once to a pc/mac and do a sync via cable in case there's an odd activity struggling to sync for some reason?

Thanks AJR.   I sync'd with the laptop, still the same but then afterwards restarted the watch and it was OK then.     I had previously restarted the watch and it didn't fix it, so the combination of the two is what made it work.

I've seen this occasionally and the clue tends to be the wired sync sometimes throws up a message along the lines of "file xx didn't sync" and so VERY occasionally (like twice in 2 years!(  I've had to manually copy the named file onto the computer and upload it manually.

My guess is, as you said, the combination broke the logjam.