This seems like the only logical explanation: The browser (or macOS keychain) autofilled the original master password, and the OP then generated a new random password (using their browser or macOS keychain) and autofilled it (or pasted it) into the “New Master Password” and “Confirm Master Password” field.
@eKKelon_R If you have not made any backups of your Bitwarden vault contents, and if you don’t have any other devices where you have previously used your Bitwarden account, then your only hope is that when you entered the “secure password generated by my MAC”, the Mac may have automatically saved this generated password somewhere. My suggestion would be to look through your Apple Passwords and Safari passwords, as well as your iPhone passwords (under “Passwords” in the Settings app), in the hopes of finding any entry related to Bitwarden there.