I ran into the same problem, Vex. BB can explain better why this happens, but I've masked out the areas that I want the original colour or my makeup shader to show through... by "mask out", I mean: I use a Blender node to decide which part of the face sees skin shader and which part allows my makeup shader or underlying colour to show without shader. It's easier to show you in Matmatic script form than even in a shader, since as you can tell, the shader is such a spaghetti of node you sort-of don't really look at it much.

ETA: I think it has something to do with GC, but I'm not sure. Corrected sRGB might not have this problem as much. CobaltKween has much more info on that.

Monterey/Mint21.x/Win10 - Blender3.x - PP11.3(cm) - Musescore3.6.2

Wir sind gewohnt, daß die Menschen verhöhnen was sie nicht verstehen
[it is clear that humans have contempt for that which they do not understand] 

Metaphor of Chooks