Azure Communication Services - proactive token refresher not executing in Android app

Question

I am building a Java Android app that uses Azure Communication Services. proactive token refresh doesn't seem to work -

before creating the CallClient and CallAgent I've got

        CommunicationTokenRefreshOptions options = new CommunicationTokenRefreshOptions(Refresher, true, Refresher.Token);
        CommunicationTokenCredential credential = new CommunicationTokenCredential(options);


        CallClient client = new CallClient();
        callAgent = client.createCallAgent(context, credential).get();

Refresher is an instance of TokenRefresher - aCallable class which I pre-instantiated and then executed once already to get the initial token (saved in the Token member).

public class TokenRefresher implements Callable<String> 

and I have a log statement as the first line of the Call method and I'm not seeing that logged regularly. I am creating an ACS token with expiry of 60 minutes, so I would expect to see a log entry at least once an hour (probably a bit more?)

my log is using AppCentre SDK, so I'm checking the cloud log and I can see it when I open the app,but not subsequently