The main issue is that Photos does not read XMP sidecars. I plan to add JPEG-XL export support in the (relatively near) future, which would also embed the keywords into the file, at which point, Photos will be able to see the keywords. The other way for Nitro to solve this is to have Nitro write XMP directly into original files (in this case, the JPEG-XL that you made in the other app). However, that is a bit longer term, as writing into original files has risks that need to be managed carefully.