I've been having the same problem.
I've realised it only occurs when I'm learning something new, but not necessarily anything particularly difficult for the left-hand. Once I get the hang of something, the numbness doesn't occur.
I assume I'm unconsciously tensing the muscles in the hand and forearm. So I think the problem, for me anyway, is purely psychological and I'm trying to be more aware of my left hand and arm and keep them relaxed (but I keep forgetting