MacOS Photos Library won't sync while on hotspot/USB tether, but Shared Albums will

Hello Arse122,

For help with syncing your iCloud Photos while connected to a Personal Hotspot, check your Low Data mode settings with the help of the article here: Use Low Data Mode on your iPhone and iPad

Also, the information below found in the article here has additional details on why a sync may be paused or aren't syncing as expected: If your iCloud Photos aren't syncing to your device - Apple Support

"What your status message means

If your iCloud Photos aren't syncing, do not turn iCloud Photos off. Syncing with iCloud might be paused for a variety of reasons, including if you turn on certain device settings or need more iCloud storage.

If you see Saving Cellular Data or Low Cellular Data Mode

iCloud Photos automatically pauses syncing until you're connected to Wi-Fi for a variety of reasons. If this happens, you might see a status message and the option to Sync Now. To sync your photos to iCloud, tap Sync Now. Or, turn off Low Data Mode. 

If you see Saving Battery Power or Low Power Mode

iCloud Photos temporarily pauses syncing if your device is on Low Power Mode or if it's low on power. To sync your photos to iCloud, you can charge your device's battery, turn off Low Power Mode, or tap Sync Now next to the status message.

If you see Waiting to Sync

If you see a status message that says Waiting to Sync, make sure that your device is connected to the internet and wait a few minutes for the sync to complete.

If your iCloud storage is full or almost full

If you exceed your iCloud storage limit, photos and videos won't sync to iCloud and your library won’t stay up to date across your devices. To continue using iCloud Photos, you can upgrade your storage plan or reduce the amount of storage that you're using.

If you see Move Photos to Personal Library to Continue Syncing

If you're a participant in an iCloud Shared Photo Library and the owner's storage is full, you won't be able to sync your Personal Library until the Shared Library owner upgrades their storage plan or reduces the amount of storage that they're using. "

Should the issue remain, check your library view and the remaining steps in the section "If your photos still aren't syncing to iCloud" from the article shared above.

Additionally, if the issue remains you can test when directly connected to Wi-Fi to further isolate, and help with the sync process.

Cheers.

Thanks for following up Arse122,

From here we'd recommend that you reach out to Apple to look into this further. You can connect with Apple directly using either Get Support or if you're in a region without direct support options, you can also Contact Apple for support and service.

Hope that helps.

Take care.