Dear Sravan.
I have the same issue as Anil6079, and many other users (who have this one, or any number of other major issues).
It's good that you defend iOS updates in general.
But I'm afraid that your 'thorough research" and your conclusions are biased and wrong...
Yes, the issue has been raised for many previous updates and therefore can not be specifically linked to the 17.4.1 update.
But no, you definitely can not dismiss it as independent of the updates...
Maybe the updates are good in themselves, but there is definitely a problem with the updating process (even if random) which makes it definitely not fail-proof and seems to occasionaly generate mayhem with the stable configurations and settings that were working fine prior to the update.
So better find a solution, rather then dismissing users concerns.
On a side note, trying to solve the problem all alone, I'm starting to suspect that the issue is not the notifications per se, but rather the "Background App Refresh".
Indeed, when an App (in my case Whatsapp) has been on the forefront, the notifications start working again.
But they stop after a few minutes, or as soon as it goes to the background when launching a different App.
Not the most robust multiprocessing I've seen...