Are you looking at the Recents album? The sorting in the Recents album is only temporary stable, but it can change over time. The reason is, that it is sorted by the date a photo has been added, and the date a photo has been added can change, when we install a newer version of Photos (as a part of a system update or upgrade) or when the Photos Library gets damaged and Photos needs to upgrade or to rebuild the library. Then the photos may be added in a different order to the rebuild library. Sometimes Photos will discover older lost+found photos in the library, that have no longer been properly indexed in the databases and will add them as new items, so we can decide what to do with them. Look for an album "Recovered items".
The date, when a photo has been taken, should not change, unless we correct it on or own. So the albums, that are sorted by the date a photo has been taken, are preserving the sort order, even after your Photos Library has been rebuild after a library repair or a system upgrade. If you cannot find a photo in Recents, look for it in All Photos, or the Days, Months, Years.