First maybe try check your DL form, most ppl DL wrongly, which over relies the muscle on arms when lifting the weight. You don't lift whole weight up with your arm, you arms are connector between your core & legs. Lift it with cores / glutes / legs.

Then work on forearm muscle more.. or use alternate grip for weak grip.

I used to have this problem with my left hand, I will switch to alternate grip (palm facing up, which relies. on bicep, usually stronger than forearm for most people) for last set in my DL routine.

After that I focus more on forearm workout, this problem is getting better.