iOS 14.4.1 notifications not working for some apps

I have an iPhone 12 Pro with iOS 14.4.1 installed. The issue is that some apps aren’t sending notifications (like Mail for emails, Habit tracker, Messages). I have paired by Garmin Fenix 6X Pro Solar watch with Garmin Connect app on phone for notifications in my watch.

Sometimes notifications get through but mostly they don’t, and it seems to be a completely random pattern.

Another instance is my dad using his iPhone 7 and Apple Watch series 6. Sometimes calling notifications get through to his watch. Mostly the watch just rings without vibration. There have also been several instances when no notifications were received on the watch. And a very few times, the watch rings and vibrates for the same contacts who called earlier when the watch didn’t ring or vibrate.

I’ve seen all notification settings on both my and my dad’s phone and watch and there are no issues with them. Am I doing something wrong?