What is the problem? Is there any Lua errors?
The border castbar for player displays with incorrect inset (and seems to be too thick)

In this example player's castbar as same size as target's one
(I may not be caused by custom UF because it's just a reskin on blizzard's one)
What steps will reproduce the problem?
Just configure player castbar

no issues with this height on target's cast :)

Seems like setting the castbar height too low screws it up. Will look into it.

Does a /reload fix the texture for you or is it always like that no matter what?

The commit referenced above in this thread is not live yet, but I think it should fix the issue completely. I will upload the fix on wednesday together with the new WoW patch. For now you can just do a /reload every time you change the castbar border to get it to update.

I think I upgrade to your last commit (1.1.8?) before seeing this message. I don't have the issue anymore, maybe because of that or maybe because of a reload, but seems cool now.