I am experiencing the same issue I had a year ago, my arms/hands go numb while riding. I had the bike professional fitted but i'm having issues. I have a strong core, I have to assume I need to raise the handles or whatever because it's not a good feeling.
Googling says it's called "Rider's Palsy".
Just noticed this post.
Hand numbness is usually caused by saddle position problems. Moving the bars higher can make it worse because it becomes easier to weight the bars.
The most common cause of hand pressure is the saddle being angled too nose-down. If you slide forward and constantly have to push yourself back while riding, this is absolutely the primary cause. Use the level app in your phone and adjust the saddle nose-up 1 degree at a time until you stop sliding forward.
The second cause is saddle too far forward. You butt needs to be some distance behind the bottom-bracket so that your pedaling can support your upper body and unweight your hands. Pedal at 75% effort and try to unweight your hands entirely. If your torso falls forward, the saddle is too far forward. I can put my hands behind my back without my torso falling forward. If your bike fitter set the saddle position by hanging a plumb bob from your knee, he is using dating techniques that at-best provides an ok starting position. That position is not set in stone. Move the saddle back in 5mm increments until you are satisfied with the weight on your hands. Lower the saddle 2-3mm for every 10mm the saddle moves back.
And the common cause that everyone deals with is incorrect hand position. The median and ulnar nerve run close to the middle of the wrist. If you rest your wrist on the bar while riding, you will get hand numbness. Make sure to turn your wrists outwards on the bike so you are holding onto the hoods more with your thumbs. Using bars that are too wide can make this issue worse.
Saddle too high can impact hand pressure, but not by large amounts. If your hips are rocking side-to-side while pedaling, or you point your toes down a ton while pedaling, or your calves don't relax at the bottom of the pedal stroke, or your hamstrings and glutes are super tight after a good ride... then your saddle is too high and you wont sit on the saddle properly and may weight your hands more. Low your saddle 5mm at a time until all of those issues are resolved.
If you do go back to see the fitter, try to pay attention to this stuff beforehand. Noticing any of these issues and letting the fitter know will allow them to provide a better more-correct solution. It's like seeing the doctor - the more good info you can provide them about how your are feeling, the more likely they will do their job well.
arty