Check if you have the validtokenissuerprefixes claim in your custom claims policy
See here for more information on this claim : https://video2.skills-academy.com/en-us/azure/active-directory-b2c/active-directory-b2c-setup-commonaad-custom?tabs=applications#add-a-claims-provider
Duplicate information here : https://social.msdn.microsoft.com/Forums/en-US/db46c12d-abc9-4bff-86da-9c24a00232dc/azure-ad-b2c-with-aad-signin-user-journey-is-expecting-wrong-issuer?forum=WindowsAzureAD
One of the values were wrong for the prefixes.