There are at least four ways to get an image from Photos to iNaturalist in Safari on Mac:
(1) Export from Photos, then in iNaturalist click Upload and then click “Choose files” and then select image files from the displayed file hierarchy
(2) In iNaturalist click Upload and then click “Choose files” and then click “Photos” in the sidebar, and then select images from there
(3) Export from Photos, then drag and drop the generated image files to iNaturalist’s Upload page
(4) Drag and drop from the Photos app directly to iNaturalist’s Upload page
If you take images from Photos directly (options 2 and 4 above), location data will not be transferred. I am not certain, but I believe this is a deliberate privacy choice to avoid users giving away location data for photos without realizing it. (Also, photos taken on the iPhone that are in .heic format don’t work in some cases; this is an unrelated issue covered by multiple other forum threads including this one.)
If you use the menu item File > Export > Export (n) Photos… to create image files (options 1 and 3 above), there is an explicit Include Location Information checkbox. If this is checked, then the location info will be included in the exported files and then transferred to iNaturalist.
There are other more convoluted ways to get photos from Photos to the iNaturalist website, but I suspect the person having trouble here is using option 2 or 4, in which case I believe this is working as intended (but also not under the control of iNaturalist in any way).