As per this documentation, I think the base URL for backend "azuread-b2c-oauth2" for custom domain should be

BASE_URL = "https://{authority_host}/{tenant_name}.onmicrosoft.com"

but not

BASE_URL = "https://{tenant_name}.{authority_host}/{tenant_name}.onmicrosoft.com"

From Azure documentation:

With Azure AD B2C custom domain the corresponding updated endpoint would look like:

https://login.contoso.com/.onmicrosoft.com//oauth2/v2.0/authorize
https://login.contoso.com/.onmicrosoft.com/oauth2/v2.0/authorize?p=