Based on your description it sounds possible that the policy was created in the regular Azure AD tenant and not on the B2C directory itself. When you added the policy, did you make sure to select Azure AD B2C in the search bar, create the conditional access policy, and then select the users and application for which the policy is applied?
Note also that Azure AD B2C is a Premium feature, so you need to make sure that the price tier of your Azure AD B2C tenant is set to Premium P1 or P2.
Another thing to keep in mind is that the policy would only apply to successful sign-ins. If a sign-in is successful, then the policies are evaluated to determine if the user should be able to access those resources. https://learn.microsoft.com/en-us/azure/active-directory/conditional-access/howto-conditional-access-policy-block-access
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