expired token when set to not expire

I'm having issues with trying to catch this exception. As suggested in another post, I'm using GetCurrentAccountAsync to test if the token is valid - I've put that into a try/catch, and the catch doesn't run! THEN in later calls I start getting the expired token exceptions. i.e. GetCurrentAccountAsync doesn't seem to care that the token is expired (or at least it's not throwing an exception for it). I'm not sure which call is actually triggering the exception (I'm trying to catch it at authorisation so that it doesn't go any further).

CODE

DxClient??=new DropboxClient(DxToken);
try {
System.Diagnostics.Debug.WriteLine("********************************** Checking that token works");
DxClient.Users.GetCurrentAccountAsync();
}
catch (Exception ex) {
System.Diagnostics.Debug.WriteLine($"********************************** Exception in changing Authorised: {ex.Message}");
return;
}
System.Diagnostics.Debug.WriteLine("********************************** Token works!");

OUTPUT
********************************** DxToken is sl......
********************************** Checking that token works
********************************** Token works!

.....
Exception thrown: 'Dropbox.Api.AuthException' in Dropbox.Api.dll
********************************** UNHANDLED EXCEPTION! Details: Dropbox.Api.AuthException: expired_access_token/