Hello, I am using this mod at Factorio version 0.18.34 and everything is alright.

The underground surfaces and the sky surfaces are ok. But like in the title, in the underground the biters keep attacking the stone cave walls and cant even make a dent on it. I remember they could brake cave walls in .17

I am using with your mod, Angels, bobs, AAI, Natural evolution enemies, cold biters, explosive biters, big monsters.

Please, is it a feature or a bug?

I don't know to be honest. I didn't know biters could break cave walls. I might look into it, but it seems weird because nothing really changed since the original port to 0.17.

I think it's because cave walls are neutral and enemies don't consider it a threat. So I guess it's a feature? I'll let you decide if it's bug or a feature :).