Sounds very interesting odd socks - i wish all my patients had your inquisitive nature!

Maybe it is a ganglion or a growth on the nerve and the nerve conduction has been altered etc etc. Or it is on the dorsal side so when in extension going into flexion, it blocks the movement but when in flexion going into extension, it is out of the way...

Get an ultrasound of the area to make sure...

Good observation though - makes me think it is more local in the wrist...