Cannot use reverse proxy with multiple custom routes

### Describe the bug
I can't seem to get the reverse proxy working as expected when I have more than one route configured in my web app.

I'm using the reverse proxy in a .NET Framework to .NET 6 port. I'm using the "out of the box" configuration for YARP and I've added two routes as follows plus the reverse proxy mapping:

```c#
app.UseEndpoints(endpoints =>
{
endpoints.MapControllerRoute(
name: "default",
pattern: "{controller=Home}/{action=Index}/{id?}");

endpoints.MapControllerRoute(
name: "EmbedScriptTrip",
pattern: "embeds/script/trips/{alias}",
defaults: new { controller = "Embeds", action = "GetTripsScript" });

endpoints.MapReverseProxy();
});
```

What I'm expecting this to achieve is:
- For a"default" route such as `/home/index` or `/embed/index` exists in this new app then use that, otherwise send it to the old app using the reverse proxy
- similarly for the `embeds/script/trips/` route, if it exists in the new app (which it does), use that, othewise send it to the old app using the reverse proxy

If I take out the `endpoints.MapReverseProxy();` then the routes all work as expected:

> `/` goes to the home page of the new app
> `/embed` goes to the index page of the new app's Embeds controller
> `/embed/script/trips/` goes to the new app's GetTripsScript action in the Embeds controller
> all other routes go to the old app

The issue I'm experiencing is that as soon as I reinstate that line to add in mapping for the reverse proxy, the "custom" route named "EmbedScriptTrip" stops working. If I have it reordered so that that route comes first, then THAT route works and the default route doesn't, and those `/` and `/embed` routes get proxied to the old app.

I've tried a bunch of alterative orders and can't seem to get to a situation where I have all my routes working AND the reverse proxying working for any routes that aren't existent in this new app .

### Further technical details

- YARP.ReverseProxy version 1.1.0
- "new app" is .NET 6, "old app" that is being proxied to is .NET Framework 4.8
- Running on Windows via the CLI (`dotnet watch run`) using Kestral