Hammers cannot mine obsidian in a 3x3, even when the block being targeted is obsidian. When attempting to mine a 3x3 of obsidian, the center obsidian block is mined as normal, but the other 8 obsidian blocks are left behind. It makes sense for obsidian to be left behind if the target block is something faster to mine, but when the target is obsidian, it seems like it should work like normal.