If running into a problem where you have multiple areas registered in an MVC ASP.NET Framework app, and you're trying to port this to ASP.NET Core MVC (using System Web Adapters and/or YARP), you may find that while the application with a single area works, multiple areas do not work (only the first area registration works).
The fix is to update your MapReverseProxy line to the following:
app.MapReverseProxy()
.Add(static builder => ((RouteEndpointBuilder)builder).Order = int.MaxValue);Or, if you're using MapForwarder:
app.MapForwarder("/{**catch-all}", app.Configuration["ProxyTo"])
.Add(static builder => ((RouteEndpointBuilder)builder).Order = int.MaxValue);I think that multiple calls to MapAreaControllerRoute (or MapControllerRoute with constraints) must increment the order of the middleware. By explicitly setting the YARP proxy to int.MaxValue, we ensure that it's last, and interleaved with the MapAreaControllerRoute.