Here is a shader solution to slant / rotate / skew the material:
Just change the material domain to UI and update the input values.
Edit: I also tried to just apply textures / mask as you did to see what happened. I managed to skew the progress bar meanwhile I am using a material for both the Fill Image and Background Image.
It doesn’t look pretty, but everything works for me. I suggest you look closer at the blueprint setup that you have. You probably have some values set causing it to not work the way you intend it to do.
