Two different things were mentioned in the original post.
1. Notifications beeing displayed too late.
To solve this, the Notify_push service must be installed and running on your server. See also here.
This also applies to the mobile apps!
2. Notifications not beeing displayed when another browser tab is active or the browser window is not in focus, and similar issues.
According to this GitHub issue, a lot of this should be fixed by now, so make sure you are up to date, i.e. on Nextloud 29.0.5, and on the latest version of the Talk server app.
For the remaining issues and the current status on them, you can follow this newer GitHub issue: No notifications from open chat when Chrome is in background · Issue #1946 · nextcloud/notifications · GitHub
However, I’m not sure how much of this is related to the mobile apps, probably not too much. So if you’re running the latest versions of the Nextcloud server, the Nextcloud Talk server app, the mobile apps, have Notify_Push installed and running on your server and are still experiencing problems, either start a new thread here and fill out the support template with more details about your setup and the exact problem you’re experiencing, or if you think it’s a bug, open an issue on GitHub in the respective repos for the mobile apps if there isn’t one already.