I had also faced the same . Let me guide you through the walkaround i had found.
The notification issue may be due to the system optimizing the app, so we have to long press in the Recents screen and lock the app in the recents. This will prevent the app from being killed.
The next step must be done in the telegram app.
Head over to settings in the app—- notification and sounds—— in the other section, you will find toggles for Keep alive service and background connection. Turn them on.
( if the issue still persists, try disabling battery optimisation in settings—— battery—-battery optimisation——in the list select telegram —— set the option to Do not Optimise)
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