When I break cobblestone with a hammer, it doesn't drop anything. And also it takes a while, so I assume it's not registering the hammer as a valid method for mining cobblestone. I have like a billion mods installed, so it might be a mod interaction. I tried the normal iron hammer and also a stone and an alumite headed sledge hammer from tinkers complement. No drops in any case. Same thing with netherrack. Sand and gravel DO work normally with both the default and the alumite hammers.

This is 1.12.2 on Linux, and I can do further testing if you need.

Wanted to drop a fix that I found with a larger modlist. If you have notreepunching, you need to add minecraft:cobblestone to the breakable block list in the notreepunching config.

i have the same problem, but i definitively don't have no tree punching installed...

it says it needs mining level stone to mine vanilla cobble but smh the hammer doesn't seem to fulfill this requirement. It only works with chisel cobblestones which don't have any mining level requirement

modlist.pdf
my modlist

i now know that GTCEu messed with the mining levels of vanilla blocks which probably causes this problem...