Apple decided to not allow the edition of metadata from a Photos extension. (This limitation has no reason and is only for macOS and not for iOS)
In order to circumvent the problem, we have designed an alternative way to do it. You can open a copy of the original image directly from the extension to MetaImage. You can then edit the metadata of the copy in the application. To finish, you only have to click on one button to import in Photos.app.