Written for OxygenOS OP3_O2_Open_7 (community build), no idea if this happens on stable builds, but it will probably start happening there.
A few weeks ago I got annoyed by a new update which introduced “a smart background app killing mechanism” as after that I stopped receiving notifications. I tolerated it for some time and, as this is some time after I returned to Telegram, I went to r/OnePlus Telegram group asking for help after official support had had no idea on what is the issue.
They immediately had the answer, open Settings, go to Battery, touch the three dots, select Aggressive doze & app hibernation and disable Enable aggressive doze & app hibernation.
When this is enabled, doze will operate more aggressively. Background apps will automatically enter hibernation.
It might sound like a good idea, but it kills notifications from mostly all apps and I already use Greenify’s Aggressive Doze which is just the normal Android doze launched in a few minutes instead of a few hours and it respects my whitelist.
This might have fixed notifications as long as I started the apps after booting at least once, but if I didn’t start the app, I would miss notifications until I started the app. I thought this was a bug and was patiently waiting for fix, until accidentally finding out what is the problem.
Open Settings again, go to Apps this time and there select the wheel next to the three dots and under Advanced open App auto-launch. Disable App auto-launch and reboot and be surprised when all notifications work again!
When you turn it on, the system will prevent these apps from launching in the background.
This might also seem like a good idea, but I still have Greenify hibernating apps that I don’t care about enough to have them receiving notifications all the time, but I would appreciate communication apps to send notifications to me regardless of whether I open them after boot by myself or not.