Hmm.
Interesting one Camo.
Have had inflammation of the tendon sheath just below the thumb, and associated pain when using the thumb to stabilise the crimp, but not quite as you describe.
Slightly strange that pinches are generally OK, but that's what you observe. Is the degree of flexion in the wrist a factor?
Just wanted to add a bit about my own experience, because treatment for the inflammation was one round of laser treatment, at Sports Injuries physio in Sheffield.
If I'm playing the guitar a lot, I sometimes get the odd "arthritic stab" of pain in the thumb joint, but again, not really as you describe.
Good luck with it!
Hopefully someone else can give a bit more insight.