This might just be me failing a numbers game, but it currently feels like the gemhunter credo's bonus of "doubling gems mined" does not activate if your inventory is full and the gem hits the ground. I don't know the exact chances of it happening (and I am probably getting trolled by RNG) but when I'm clearing my inventory while mining, it procs fairly frequently, and if I'm clear cutting a mine I have never seen doubled gems hit the ground.
Can anyone else confirm it's working as intended? Will update if it happens to me.