House says it’s corrupted, isn’t.

I was trying to make a complete crafting station including a demon altar, so I built it underground in the corruption. I added the three houses for the NPCs who wanna be underground, but it says they are corrupted. No problem, I look up the “house”section on the wiki, and dig out a giant square of the same size. I fill it in with ebonstone bricks to make it look ok, but it still says it’s corrupted. I checked the wiki and tested at my forest base, ebonstone bricks does not effect the evil score. Tried restarting the game, didn’t change. So what gives? Why does it say corrupted and it isn’t?