@Kilian BONFILS Thank you for reaching out to us.
Azure AD B2C uses Azure AD B2C Premium P1 license, which is different from Microsoft Entra ID P1. A subset of Microsoft Entra Conditional Access features is supported with Azure AD B2C.
However, If you want to use Identity Protection, risk-based Conditional Access policies, and any future Premium P2 capabilities with Azure AD B2C, you need to upgrade your pricing tier to Azure AD B2C Premium P2 license in your B2C tenant.
https://learn.microsoft.com/en-us/azure/active-directory-b2c/supported-azure-ad-features