Lightroom CC 1.1 for MacOS has fixed a few sorting issues by now syncing a CUSTOM SORT from Lightroom Classic. However, if the custom sort is changed in L. Classic, this does not sync across to LR CC unless the sync option is turned off on the L. Classic Collection, thus deleting the album in LR CC. Resyncing the Collection then correctly does the custom sort in LR CC. This should not be necessary. There is still a major omission in LR CC in the sort options: cannot sort by file name, even though this can be done in the iOS app. I often rename photo file names into a sequence in a Classic Collection as a means of ensuring the custom sort is durable.