"Unfortunately, Photos will not be able to combine the two components properly, if you already imported one part of the pair, for example only the JPEG (or HEIC). When you then download all files, you will be seeing the components in photo as a video clip and a still frame. This can happen, if you have My Photo Stream turned on. My Photo Stream cannot transfer videos, and so it will only transfer the JPEG. When you then later try to import photos manually, you will get split pairs of Live Photos. My Photo Stream is preventing the proper import of Live Photos, while it is enabled."
Thanks for your feedback.
But I've NEVER already imported either part of pair of 'Live' photos separately.
So again, I'm reporting this as a new unwanted behavior, that I noticed ONLY AFTER recently updating my iPhone to iOS 16.2, and my iMac 2019 to macOS Monterey 12.6.2.
I've always intentionally used the 'Import All' feature of Photos to import everything from my iPhones while connected via a genuine Apple USB-to-Lightning interface cable, never selecting and importing only .jpg files or only .mov files.
To my knowledge, until iOS 16.2, the two separate files for each 'Live' photo stored on my iPhone were never listed separately when viewed while the iPhone was attached with a genuine Apple USB-to-Lightning interface cable while using macOS Photos.
I explicitly exclude Photos from all AppleID and iCloud settings on every Apple device.
'My Photo Stream' has never been intentionally enabled across my universe of Apple devices.
Could 'My Photo Stream' have been unintentionally enabled by the most recent iOS and macOS updates?
Unlikely, since I still use only the 'free' 5GB iCloud account. My iPhone contains 75GB of images, and my iMac Photos library is over 400GB, so there's simply no room available in iCloud.
I routinely use Finder to back up my family's mobile Apple devices to two Macs: the 2019 iMac Intel i5 21.5" 4K (mentioned above), and a 2012 Mac mini Intel i7 running macOS Catalina.
Then I upload whatever the heck it is that iOS Photos captures to the latest applicable version of Photos on the macOS devices.
Photos on macOS Catalina HAS NOT exhibited the unwanted behavior of importing 'Live' photo JPGs separately from MOVs.
I've always intentionally used the 'Import All' feature of Photos to import everything from my iPhone when connected via USB-to-Lightning interface cable, never selecting only .jpg files or only .mov files.
Seeing no resolution at this point, I'm going to purge my current Photos library on my 2019 iMac and all the corrupted backups, downgrade macOS back to Catalina, and then re-import my 400GB of digital images into the Catalina version of Photos, when none of this duplicate file nonsense was visible.