onTrialStarted callback not triggered as expected

[hydravion]

I can trigger the callback in a strange way, but it does not seem to work under normal conditions.

I have set up event listeners in the service worker like so:

const extPay = ExtPay('hogwarts-graduate');

extPay.onPaid.addListener(executeOnPaidCallbacksInEachTab);
extPay.onTrialStarted.addListener(executeOnTrialStartedCallbacksInEachTab);

extPay.startBackground(); 

The onPaid callback works well, however, the onTrialStarted callback is usually not called.

For context, I have started many free trials (which show up on ExtensionPay's website) with my developer account in test mode.

Normal scenario where the callback is not called:

1. Install the extension
2. Sign up for a free trial (dev email in test mode)
3. Callback is not called

Strange scenario where it is called:

1. Install the extension
2. Sign up for a free trial (dev email in test mode)
3. Callback is not called just yet
4. Pay for the extension (dev email in test mode) and keep the popup open
5. Uninstall the extension
6. Reinstall the extension
7. Sign up for a free trial again (dev email in test mode)
8. Callback is still not called
9. Click on the popup from earlier (the one where the dev can switch between user statuses)
10. Reload the popup with F5
11. The onTrialStarted callback is called

I have noticed that ExtPay doesn't receive the trial-start message. However, it does receive the fetch-user message.

I suspect that refreshing the popup triggers the poll_user_paid function which in turns calls the fetch_user function and that it will trigger the onTrialStarted callback from there.
I think that the onTrialStarted callback does not work otherwise because ExtPay does not (seem to) receive the trial-start message from ExtensionPay's server, even though it does receive fetch-user.

To check which messages it received, I simply added breakpoints in key places in:

if (typeof window !== 'undefined') {
    window.addEventListener('message', (event) => {
        if (event.origin !== 'https://extensionpay.com') return;
        if (event.source != window) return;
        if (event.data === 'fetch-user' || event.data === 'trial-start') {
            runtime.sendMessage(event.data);
        }
    }, false);
}

and

        startBackground: function() {
            runtime.onMessage.addListener(function(message, sender, send_response) {
                console.log('service worker got message! Here it is:', message);
                if (message == 'fetch-user') {
                    // Only called via extensionpay.com/extension/[extension-id]/paid -> content_script when user successfully pays.
                    // It's possible attackers could trigger this but that is basically harmless. It would just query the user.
                    poll_user_paid();
                } else if (message == 'trial-start') {
                    // no need to poll since the trial confirmation page has already set trialStartedAt
                    fetch_user(); 
                } else if (message == 'extpay-extinfo' && management) {
                    // get this message from content scripts which can't access browser.management
                    return management.getSelf()
                } 
            });

Would anybody know where to go from there? What am I doing wrong?