According to this documentation, the on-behalf-of flow is not supported in B2C:
Web API chains (On-Behalf-Of) is not supported by Azure AD B2C.
Many architectures include a web API that needs to call another downstream web API, both secured by Azure AD B2C. This scenario is common in native clients that have a web API back end, which in turn calls a Microsoft online service such as the Azure AD Graph API.
This chained web API scenario can be supported by using the OAuth 2.0 JWT Bearer Credential grant, otherwise known as the On-Behalf-Of flow. However, the On-Behalf-Of flow is not currently implemented in Azure AD B2C.
Can't I just pull out the JWT from the first Web API request and pass it along to the next Web API? I know technically, I can, but is there a reason I wouldn't want to?
This approach would only work if both Web API's are configured for the same B2C App. Maybe that is the difference. Is the documentation referring to 2 separate B2C apps maybe?
Reference: Access the JWT bearer token when using the JWT middleware in ASP.NET Core