It could be that for whatever anatomical reason your shoulder does not like to hold weight at that angle.
One way to try a different angle would be to get a different stem. Going shorter and higher is usually the first thing fitters try when there is hand or arm numbness.
You could also try handlebars of a different width, but that's less likely to make a difference and more expensive to switch.
If you have the chance to ride a different bike that fits you, but with different geometry, for 12+ miles, that might lend some insights and not require buying parts to experiment with.