Email Encryption: Disable MS365 logins and force OTP codes via email

Currently, all outgoing encrypted emails are required to use MS365 login for *any* MS365-enabled tenants.  This is a problem, because we can't use this with external tenants for some reason - it's not clear why, but there's a lot of external tenants who are failing to authenticate.

We know that this is sometimes due to internal team/distro lists at external tenants not being able to login/auth.  However, *historically*, we were able to select "Login with Microsoft Account" or "Receive OTP code via email".  In the modern Purview environment, it doesn't seem that changing settings in OME settings or IRM settings will allow you to actually disable social logins and force OTP codes as the only mechanism.

Whether this is due to the way Purview policies are set up for "Encrypt" profile or not, I don't see a way to change this and force *only* OTP.

Is there any way to disable all social ID login **including MS365 logins** for encrypted messages, and force only OTP codes?