Last I checked, iCloud Photos does not sync masters/originals to the cloud. It uploads these only once and then considers the cloud to hold the "truth". What is on your Mac is considered a mere local cache / disposable copy.
Geotagging these files does not cause iCloud Photos to upload the changed files. Thus all appears good on the Mac where you geotagged the images. Should you ever download the files from iCloud to a new Mac you will find that these are not geotagged. Or most are not geotagged. Some may be if you managed to geotag them before iCloud Photos got a chance to upload them.
iCloud Photos also has the option to "optimize storage". This has it remove local files that Apple Photos does not currently need. I.e. it will delete the geotagged copies and at a later point download the files lacking geotags from iCloud Photos.
That's why I recommend users of iCloud Photos to geotag photos before loading them into Apple Photos. E.g. by using the camera import feature in HoudahGeo to copy images from your camera onto your Mac first. Geotag the photos and only then add them to your Photos library. Thus iCloud Photos will only get to the images after these have been geotagged. This way geotags get backed up to iCloud. Also: you can skip the "notify" feature in HoudahGeo. Apple Photos will load location information from the EXIF/XMP tags.
I have logged a feature request with Apple to actually sync changes to image files up to the cloud. I don't think this has been addressed in macOS Mojave - but have yet to check.
If you want to have geotagged versions of your existing photos uploaded to the cloud, you will need to export remove them from the Photos application. Geotag the files and then re-import them. In the process you will lose edits (metadata and retouching) you made in Photos.
Alternatively, you can geotag the files on your Mac and create a backup other than iCloud Photos. You will still not have geotagged photos in iCloud, but have safe copies of your geotagged photos should your hard disk fail.
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