This test ist conducted with two installations of NC 24.0.2, both used by the same desktop client 3.5.1 on MacOS (but the problem is with Android clients as well).
php occ notification:generate <myself> <sometext> on the first NC installation, the notification will appear in the web app as well as in the desktop client as expected.
When executed on the second NC installation, only the web app will receive the notification after some seconds. None of the NC clients gets any notification.
File syncing of the NC client works fine with the second NC, which is using an LetsEncrypt certificate (just as the first NC system). I found that NC#1 has the event_update_notification app enabled as well, which was missing on NC#2 for some reason, but installing+enabling didn’t help