Placing your weight and gripping too much plus minor setup done wrong has the effect. I struggled with this when I first started long climbs. I started to consciously adjust my posture and weight placement but it took a long while especially the finger numbness. On flats, on long ride, not an issue. Funny how these things manifest.
What did I learn is that numbness, slight pain in one knee, etc were all good indicators that something is amiss. And that it can eventually can be fixed.
Good luck and loads of patience.