This is with normal borders, but I think I may have found a cause. When I switch to a different profile the same bar is getting animated scaled in with onupdate and setscale. And during that time I think i'm doing SetBackdrop(). I may have to pause the animation set the backdrop and resume.
This scaling animation been a thorn in my side cause I cant use blizzards built in one to do it
Took me days just to get it working, without bugging out other parts of my addon.