Got a 40+ character, a dark elven sneaker/archer.

Noticed his archery is a red 15.

However, if I zoom in on a particular perk, it shows it at the correct value, 86.

He's not diseased. I've taken off every piece of gear, slept 24 hours, with no joy.

The only recourse I can see is to skip back to an earlier save (where it's not bugged), I'll lose a day or so of gameplay.

It's a bug mate. Raise the skill level by 1 and it'll go back to normal