AD B2C - Custom Policies - Evaluate custom extension

Probably this link can clarify your context and answer your question: https://learn.microsoft.com/en-us/answers/questions/2017886/mfa-enforcement-oct-15-2024-azure-b2c

I cannot see the rest of the user journey from your snippet. So, assuming that the Social Account is enabled, the MFA can be hosted by the Social Identity Provider (or other OIDC party), then can be logically desume that the handling of the value for the custom claim "AlwaysMFA" is strictly related to your user flows (in the meaning of consideration of all the flows that a user can perform in your Sign-Up/In).

Example: if you have the Google sign-in enabled, then if the user has the MFA set over there your enforcement can cause a bad UX experience, otherwise not.

As suggestion of mine, keep the management of this value in the "Edit Profile" because only the user knows how many MFA is enabled over his/her/its accounts.