Microsoft MFA has the option of being set up (by admins) with either standard TOTP or with their proprietary algorithm.
If the admins for the realm you're trying to use have chosen the proprietary one you need to use the Microsoft Authenticator app. Regular TOTP generators will accept the code but the code they make won't work.
Can the regular Bitwarden generator make good codes? If so, it means they figured out (or were told by Microsoft) how the proprietary algorithm works. But since this standalone app is open source they couldn't add that algorithm to it.