Retaining current metadata after editing in Photoshop.

When I give someone a shared link to one of my albums & he downloads a photo from that shared link to edit it in Photoshop, then returns that edited file to my Lightroom, it doesn't include the current Keywords for that photo. In other words, it seems like any Keywords that have been added/changed since the photo was first Keyworded are not included. New photos are immediately added to the shared link & changes to the caption happen immediately, but not Keywords. 
Can you please help me with this.