It looks like you are reporting this as a bug with the game with the Demon Altar appearing in the Crimson World. I'd recommend reposting this as a Thread in the PC Bug Reports Section: https://forums.terraria.org/index.php?forums/pc-bug-reports.113/
This Thread is dedicated to Bugs with the Forum itself and Forum System rather than the game. Bug Reports which I linked here are for Bugs about the game.