Yes, the AdWords API .NET library assumes that UpdatedOn uses the same timezone as the server timezone. So if you use UpdatedOn in UTC and the server timezone is UTC+1, the library would always think that the token is expired.
Could you file a feature request on
https://github.com/googleads/googleads-dotnet-lib/issues? I think the dates in general should be in UTC in OAuth classes; this is something we can address for an upcoming release.