Had the same issue. The saddle I was riding was a "comfy" one. Lots of padding etc. Turns out the padding distributed pressure all along the sit region, compressing everything.

Each time I get a saddle it becomes smaller and harder, but at the same time I find it more comfortable. No more numbness. I'm riding a specialized toupe right now. The majority of my time I am on my sit bones and only my sit bones.

You said it isn't a saddle issue, but you also asked what other people have done about it. However, since you go numb sitting in a chair, go see a doctor cause most of us aren't and we can't diagnose or treat you if it isn't a saddle/fit problem.