If what you see is the same as Eshel, I will ask you the same question: shouldn't you apply face-remapping to the 2 new faces instead of the 'header-line' face from which they both inherit? What happens if you do define remapping for those two new faces?

My specific problem does not occur if I remap the two new faces. Why would I need to do that though? Both inherit from header-line, so if I wanted to change both, I would naturally change the base face.

Furthermore, this problem only happens *once* per Emacs session. After that, I cannot seem to reproduce it again until I restart Emacs. All of this points to a bug, in my opinion unless header-line is considered deprecated or somehow falls into a realm of not being able to remap for some reason.

Thanks,