✅ This was happening to me and through some testing I think I have found the issue.
I edit my photos via VSCO (free) on my iPhone X. After editing, when saving to the camera roll, the photos would appear for 2-3 seconds then disappear. Thankfully, the edits are stored in the VSCO studio and not lost.
In trying to solve this, I found that even when exporting the edited photos directly to my Mac via AirDrop and manually adding into Photos - the edits were mysteriously stripped of all edits and returned to Live iPhone photos within that same folder. I also found that this was only happening with photos that I took with my iPhone and not photos that were sent to me via text, etc.
My only workaround so far has been to directly export all recent VSCO edits to my Mac via AirDrop, then edit re-save with Photoshop to strip metadata from the images and manually re-add to Photos. So far all images remain and iCloud has backed up Photos on both Mac and iPhone.
The core issue has to be with third party application exports and file naming. When looking at the original photos and edited photos side by side, it appeared that the filename was not changed correctly therefore Photos would strip all edits and return the photo to its original state.