Locally saved photos disappear without deletion on iPhone

The same photos disappeared from both phones, is that correct? That would be expected, since iCloud Photos would sync the deletions.

Possibility 1: Are these two iPhones the only devices connected to the same iCloud Photos library? Does anybody else have access to either of them or any other devices connected to that library who might have deleted those photos from those other devices connected to the same library?

Possibility 2: When you say "all the photos which were locally saved on my phone have been disappeared", what did you mean by "locally saved"? Were these photos pushed manually to your phones by wire from a computer before you enabled iCloud that disappeared at the time you enabled iCloud Photos? That is a specific instance that is known to cause loss of photos, but the Photos app should have warned you about it and Apple Support should have known about that explanation.

pankajmore999 wrote: … How photos can be disappeared without deleting?

As far as I know, they can't. Surely, having this experience, you have a backup, is that right?

How to back up your iPhone, iPad, and iPod touch - Apple Support