Maybe the files have lost their metadata in some workflow?
The key metadata item in Live photos is ContentIdentifier. It has to be identical in both the image and the movie. Even the file names do not have to match -- Photos.app shows only the imported image's filename. Also the matching movie can be from a completely different footage such as .mp4.
======== ./IMG_2158.JPG
[Apple] ContentIdentifier : F906FE4C-506A-4F85-B9BE-5F862ED06C90
======== ./IMG_2158.MOV
[Keys] ContentIdentifier : F906FE4C-506A-4F85-B9BE-5F862ED06C90
So if all metadata is stripped from from that pair and renamed as a.jpg and a.mov, they can be made a Live Photo pair again:
Copy Apple makernotes from some other Live Photo image -- IMG_2160.JPG in the example below (currently exiftool can write Apple makernotes in images only as a block, not as individual tags):
exiftool -overwrite_original -TagsFromFile IMG_2160.JPG '-All:All<All:All' a.jpg
Then copy Apple:ContentIdentifier from either image (IMG_2160.JPG or a.jpg) to Keys:ContentIdentifier in a.mov:
exiftool -overwrite_original -TagsFromFile a.jpg '-Keys:ContentIdentifier<Apple:ContentIdentifier' a.mov
exiftool -a -G1 -s -ContentIdentifier .
======== ./a.mov
[Keys] ContentIdentifier : DBDA7F31-8133-4E79-8A2E-35624C26EABB
======== ./a.jpg
[Apple] ContentIdentifier : DBDA7F31-8133-4E79-8A2E-35624C26EABB
macOS 14 Sonoma Photos.app imports both the edited "a" and the donor "IMG_2160" pairs OK.
...
Android "Motion Photos" are JPEGs with an movie in the metadata embedded in the same file which is more messy.
