It forces you to install the vendor's proprietary authentication app on your mobile device.
If the site or app poses no choice, just say that you want to use their "Proprietary Authenticator" and you just continue with your own password manager.
It works for me with 1password. As a sanity check too see if it works; you always have to use a first OTP to activate the multifactor authentication.
OTP lets you use your own app.
Notification-based OTP requires a proprietary app.