Oh fuck you ended up with the Forge ID corruption Bug, you installed one random Mod, does not matter WHICH mod it is, and suddenly forge really super duper fucks up ALL the block ids of the world. The only way to fix that is to remove the most recently installed Mods.
This Bullshit happens to me CONSTANTLY on my Test World and its always only whenever i install a new Mod, no matter which Mod it is…