A couple of guys I ride with suffer from hand cramps, usually after long descents or progressively getting worse when we do long bike rides with multiple descents.
I’d always assumed it had something to do with them either gripping the bars too tightly or having too much weight on their arms.
One of them has just changed to a wider bar with some new grips so it’ll be interesting to see if that helps. The other guy’s new to mtb so it’ll be just as interesting to see if he stops complaining as he does more.
Out of interest, have you been able to work out whether its caused by a particular type of riding? i.e. very technical descents or very fast and you’re braking a lot?