I have been using this filter to identify photos that are not yet in any album and it has suddenly stopped working. LR seems to be perfectly synchronised between the two computers with identical numbers of photos and album structures. Until recently I was using this filter to identify orphan photos and then place them in an album. The filter has stopped working and while the Macbook show over 35,000 photos not allocated to any album, the imac only shows about 1,500.
I have reset preferences, reinstalled and I have been in touch with Adobe support who have re-synched the entire 77,000 photos on the imac. This has just finished and the problem persists.