I had the same problem and your solution worked. It may have something to do with this warning I get:
Warning NU1605 Detected package downgrade: System.Text.Encodings.Web from 5.0.1 to 4.5.1. Reference the package directly from the project to select a different version.
MyApp.Android → Auth0.OidcClient.Android 3.2.3 → Auth0.OidcClient.Core 3.2.3 → IdentityModel.OidcClient 4.0.0 → IdentityModel 5.1.0 → System.Text.Encodings.Web (>= 5.0.1)
MyApp.Android → System.Text.Encodings.Web (>= 4.5.1) MyApp.Android
However, I cannot seem to get the authentication result from the authenticate call. I hope that’s a configuration error