That is exactly the symptom I had, middle finger and thumb acting up. I was 38 when I identified it, and I had (at the time) very high grip strength. For me, it was the prolonged time holding 1 level of grip force in the same position for a long time + vibraation.
My H2 doesn't have the same issue, the return spring is so light I can just rest my hand in place and it stays open with an open hand.
Moving bars up/down/forward/back will reposition the arm, possibly alleviating the strain on the inner-forearm (where grip power is generated) through angle manipulation, as mentioned.
You'll find a solution, the bikes too **** good to swap out over such a small thing