It's not a bug, just a byproduct of how the game handles biome generation. I get that feedback alot but until now I have just accepted it to work this way, because changing it would require quite a bit of work to keep track of anvil locations in the world and then do some distance checking when spawning them. In addition to that, lots of testing to make sure I didn't mess anything up with the change.

Right now the system is really simple, I'm just inserting them into the pool of special rooms (like the one in the mines with a lot of potions on shelves) with a certain probability to get placed, the rest is up to the game. Sometimes the game just decides to place 3-4 of the same tiles very close together.

Theoretically the same should also happen with the vanilla rooms but I haven't noticed it that much.

Edit: Regular special rooms have a much lower chance to spawn in general that's why it's not likely to happen with them.