Star rating and keywords are removed when I add photos to camera roll

When I export my photos to camera roll on the iPad my keywords and rating are removed. I added the watermark and this is indeed visible. When I re-import from camera roll in Lr the keywords and rating are gone. With an external metadata app I can see there are no keywords in the camera roll. I added an other keyword with this app and imported again in Lightroom. Now this keyword is present. In the Lr settings I included to share metadata. The standard meta data are indeed exported. Why are keywords and ratings removed?