This is unfortunately a bigger issue
We will definitely fix it, but unfortunately it requires a world conversion for existing worlds... this issue happens when mining in forest areas, but otherwise it should work a lot better... it's an annoying issue, and we will fix it ASAP!
To be honest, this is happening to me underground in a high mountainarea as well. But i assumed its (like in Java) a matter of a tiny lack. Sometimes when i hit the rock with my pickaxe its making the correct sound but the rock is not changing shape or gone, no matter how often or how long i hit it. Neither do i get any material. But when i change the direction im looking at a tiny bit its working and when im going back to the original direction also the "resistant" rock can be pickaxed as well.
As i have activated the fast harvesting (schnelles Abbauen) option i though its a lack that occurs when mining too fast. Next time its happening ill send a report