This may be because of how push notifications work on those platforms, that is applications have to contact a centralized platform (Google/Apple) to notify user

This may be because of how push notifications work on those platforms, that is applications have to contact a centralized platform (Google/Apple) to notify users instead of relying on traditional methods. There are standards in the XMPP ecosystem (XEPs) to do that right though it's not enabled by default on most servers yet (see prosody's mod_{muc_,}cloud_notify).