I tested this and could not reproduce the problem, filtering by score = "rejected" works as in prior versions. I am able to see only rejected photos if I choose that. I use this as part of photo culling fairly often, to delete the photos I do not need.
In this version they expanded the light table filtering capabilities to make it more flexible (you can filter by many different metadata fields now and create filter expressions, I think) , so you may want to first click on "reset filters" and then rebuild your filter.