I was setting up the UI and when I was finished setting up on my warlock I noticed that both the Pet Unitframe classic skin wasn't even showing up.
I also hit F1 to target self and the classic skin for the TargetTarget frame is warped. when I enabled the classic skin it didn't show up normally but it was more stretched vertically.

I will test for other frames too.

I seem to be having a similar issue - just updated to the newest version and TargetTarget and the frames in the top left don't have the classic skin

the classic skin is still under development - will not be final till version 6.1