Everytime a user logs in, he is forced to enter the OTP. MyBB by default allows sessions that do not expire. Thus if you are logged in on PC-A, you will remain logged in unless you switch your browser, clear your cookies, etc. OTPs were implemented to make use of this feature, ensuring that user has to enter his OTP only when he logs in from a new device or browser.
2FA has no advantage over OTPs except that the passwords will arrive late if you have a slow mail server.
Ah, okay then. I just thought it was some kind of a bug (Hence the "I guess") since useres weren't prompted for it.