During OS updates, android can occasionally reset some notification settings and app settings under the guise of security (I actually think this is a decent “damned if you don't” sort of practice on the Android project's part, but we found an unfortunate side-effect). Because Element intentionally does not use the standard GCM (Google Cloud Messaging - I still like to call it this because Google is in the name, but it's currently called Firebase Cloud Messaging - FCM) for push notifications, it is necessarily at odds with what the Android OS “wants”, and we, as wise and knowing defectors from what Android “wants” in this case, must configure our software properly. I'm assuming the issue I personally experienced was likely also the issue others experienced, which was disabling battery optimizations (basically, telling Element in Android's settings that it is allowed to run even when the phone's screen is off, so that it can receive push notifications whenever someone messages you). 

​There is a very handy feature in Element now that you can use to troubleshoot this! In the upper left-hand-corner, select your avatar icon > Settings > Notifications > Troubleshoot Notifications (at the very bottom), then take note particularly of the battery optimization check item, and fix it if it fails the troubleshooting test (and any other things that might influence whether Element works as you'd like/expect). I happen to have one item (Custom Settings) that is in a (!) state but it doesn't affect me. I will update the Wiki soon with a nice write-up on how to fix this and keep it fixed, as it is an Android issue, not an Element issue.