Notifications are unreliable

[Destroyer]

Steps to reproduce

I am running a homeserver for friends. I am always getting all message notifications on my phone just fine. Friends do report however, that they sometimes miss a notification

Pixel friend recieves about 30% of the notifications.
Xiaomi friend recieves about 20% of notifications (he's on matrix.org though and i am sending him messages from my server).
Third friend does have to clear cache sometimes which resolves the problem fully (OnePlus)

Troubleshooting in the mobile app ends up all green
Clearing cache sometimes solves the problem for a little, then it reappears

Outcome

What did you expect?

Notifications to be reliable

What happened instead?

Notifications appear 20-100% of the time

Your phone model

Pixel 3a / Xiaomi / OnePlus

Operating system version

Android 12 / Android 10 / Android 11

Application version and app store

1.3.12-1.3.15

Homeserver

matrix.vcklan.cz (synapse v1.49.2), matrix.org

Will you send logs?

No

Are there any options for me to debug such issue?

[ouchadam]

I've also seen similar quirks with the notifications but then other times they would work perfectly, could I ask that your friends try going into their room settings -> notifications and selecting none then back to all messages

if this solves the issue it means our push rules are falling out of sync somewhere~

[Destroyer]

Tested with Pixel friend - not sure if it 'solved' the issue, it was reliable for about 6 hours before first missed notification

also none of us uses fdroid versions, everybody got it from google play

[ouchadam]

centralising the issue to #5038