Hi,

https://learn.microsoft.com/en-us/azure/active-directory-b2c/saml-service-provider-options?pivots=b2c-custom-policy#configure-idp-initiated-flow

""We don't currently support scenarios where the initiating identity provider is an external identity provider federated with Azure AD B2C, such as Active Directory Federation Services or Salesforce. IdP-initiated flow is supported only for local account authentication in Azure AD B2C."

This is for IDP Initiated.

Not aware of any restrictions for SP Initiated.

Rory