I have had trouble getting solid fog to work in end game content, as it appears to be giving enemy mobs a WILL save against ALL of its effects.

Solid Fog is a physical effect, not mind-affecting in anyway. It isn't giving the opportunity to dodge something or endure a physical effect, either, so reflex and fortitude also seem inappropriate for use here.

I'm not sure if the other fogs have similar saves right now, but if they don't, why solid fog?

As much as I hope this is a bug, it seems unlikely that random code interactions would cause legible text and a related effect.