Dang. All the easy possibilities are being eliminated. Which isn’t the greatest news I’m afraid.
The only other thing you can do ahead of hearing back from Support is examine the belts up close and personal. As much as you can, sometimes it’s hard to see the whole thing. (And make sure the machine is off before moving the head around to see it.)
Check to see if the tension is similar on both the left and right side belts, and check the one the head travels on. (The X axis.) Make sure there isn’t a worn tooth somewhere that might make the head slip during rapid tight movements.
I might be completely off base…this is just searching for clues, and it might not be the cause of whatever those skips are. Support has a lot more diagnostic information available to them.
But it’s something you can take a look at while they diagnose.