Have you tried rotating the handlebars? Look to see how much yhour wrists are cocked. Ie, are they straight like you are throwing a punch or rotated forward or back. If they are rotated, trying rotating the handlebars maybe 10 degrees to bring them straighter, then ride them that way for a while and see if things are better. If they are better, but you still have issues, try another 10 degrees. If worse, go the other direction.
I've been having hand numbness on a bike i have just set up. I am about to angle my brake hoods down a little to see if that will make a difference.
Ben