Starting in Android 8.0 (API level 26), all notifications MUST be assigned to a channel. Disabling Low Priority Notifications channel will prevent unimportant notifications from appear in the notification bar.

Please check Low Priority Notifications channel is disabled

Go to phone Settings / Apps / Choose our app

phone Settings

 

Tap App notifications

App notifications

 

Move the switch to the left until it turns gray

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Permissions

Battery Optimization

Do Not Disturb

Low Priority Notification Chanel

Third Party Apps

Unrestricted Data Access