Hi, I have the same problem. Update the latest version of OneSignail and the notifications do not reach the recipients. When I publish a new post I get the following message.
OneSignal: publishing post. Are you sure you want to notify your subscribers?
Note: cancelling will still publish your post but won’t send notifications
Whether you accept or cancel it, notifications do not arrive.
It should be noted that, I have the latest version of WordPress 5.0.3 installed.
Also before upgrading onesignal to its latest version everything worked fine with notifications.