Check your trash bin, if photo is backed up into Google photo, just redownload into your phone.

You can download multiples photo on Google photos.

If you didn't modify the photos' date, it will be in sequence.

Google Photo have option to delete your device photo after backup and sync, did you missclick it?

Wow, I wondered if I was alone in this. I thought my sd card was corrupted or something.
I stored almost all my lifetime of phone camera photos in the sd card from 2008 onwards in my sd card, and I also found out a few months ago, photos from 2013 to early 2020 disappeared. My sd card which is usually 90-95% full suddenly became 40% full only.

I also suspected Google photo but, I didn’t use the save space option. If yes how could I only clear a chunk in the middle?

But if it was my sd card damage / corrupt how could TS have 2 of the same problem near the same time?

Do you have other backup apps? I have synology file doing similar thing, backup and a clear but it was rarely working. I also have onedrive. But it was full and hardly working too.