I've done all your steps, Kalypso, in terms of blacking out the bangs part of the transMap, except instead of using Blinn I used Anisotropic, which worked well on another figure but not so great on this one. But anyway, same artefact... except not quite as noticeable over the lower part of the face, but you can see clearly where the bangs mesh stops in the front of the hair near the hairline.
I rendered this image in PP2010 (my new favourite toy!!), transMaps set to GC of 1 - which turns it off - and texture-filtering also to None, using a skydome of BB's and 3 infinite lights. Why 3? No clue... I should be using only 1 or 2 at most, but oh well. RT shadows set on Light 1.
The anisotropic reflections conflict with the burnt-in specular - wish there was an easy way to re-texture hair to get rid of that. Is there like even one hair texture resource kit that doesn't "feature" burnt-in specular? A bit like trying to find eye colour textures without burnt-in reflections. You have Buckley's of finding that, too.
In this hair:

... the specular is more spread-out and not so obtrusive so that you can do your own specular stuff and have it look reasonably natural... eventually. Still need to play with this more.

But yeah... here's the render of SF...

Same issues are you described. Oh, and that's with raytracing set to off for the hair.

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