I installed PM 6, and immediately ran into a problem with not being able to use the 0-5 keys to apply star ratings to photos (the preference is set up for this to work). I also cannot use the context menu to apply star ratings. Interestingly, trying to apply a star rating does create an XMP file for the photo, but the star rating is not applied because it does not appear in the Contact Sheet and filtering by star rating acts as if the photos do not have a star rating.
So I quit PM 6 and went back to PM 5. I can apply star ratings now as before, but when I export to Lightroom, the star ratings do not come across. I am not applying any metadata presets on import in LR, and my settings in PM are set up as specified for this to work.
Did installing PM 6 install some library that has screwed up exporting star ratings to LR for PM 5? This used to work perfectly in PM 5 and I have changed nothing in settings or elsewhere. One possibly salient fact: I did recently upgrade to Mac OS 10.14.4.