I use My Phone Explorer. Allows me to download (cable, Wi-Fi or Bluetooth) pics etc to my HD. And lots else - much better IMHO than the Samsung app. You could, of course, put them on any cloud you want. I also use it to send SMS if at the PC. Much easier with a proper keyboard.
This is my second Samsung. Not had any pics disappear.
Have you tried taking the SD card out of the phone, putting the card in a adapter (or SD card slot on some laptops) and checking what's in the photo folder/directory.
If its photos being taken by the phone they probably are in the folder/directory DCIM/Camera
It may be that the Gallery App is playing up rather than the photos being deleted.
If you are familiar with FTP perhaps put a FTP client on the phone (SwiftFTP) and use a FTP program on your computer to access the contents of the SD card.
If the option is set, Google Photos on your phone will automatically upload photos in the background to Google's cloud. From time to time utilties on the phone, such as Google Files, will pop up with an offer to create space on the phone which often includes an option to delete your local photo files leaving just the cloud photos. Perhaps this happened to you.
Unfortunately you can't tell where a photo is stored from the Google Photos app. However you can download the photo again to your phone by signing in to Google Photos on a PC browser but I don't recall the exact steps.
On the free cloud storage service Google now applies compression when storing in the cloud, although last time I checked it was remarkably little. You were able to disable this in exchange for Google providing less cloud storage but Google keeps modifying the rules. See:
https://blog.google/products/photos/4-things-know-about-google-photos-storage-policy-change/ Once this is all working, you can choose to completely disable Google Photos on your phone. Alternatively you can ask it not to upload in the background. I also disable any file cleaning utilities and would never accept their suggestions to remove downloads, duplicate files, unused files, etc as I would lose important data.
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