An honest question, not trying to be argumentative. If Signal server is evil and starts to MITM a communication between two parties, replacing each of the part

An honest question, not trying to be argumentative. If Signal server is evil and starts to MITM a communication between two parties, replacing each of the parties keys with their own keys and decrypting in the middle, both of the participants will also just get warning “the other party changed their key”, right? That was the gist of the issue of yesteryear with Guardian and WhatsApp, right? How is this in practice different? User see a warning and ignores it (as people change their phones thus their keys all the time). I’m not trying to be snarky but actually asking a question