The View > Sort > Last Comment Time is incorrectly greyed out for a synced collection, though I'm able to select Sort: Last Comment Time in the bottom tool bar. (LR CC 2015.7 / OS X 10.11.5.)
Screenshot showing View > Sort > Last Comment TIme incorrectly greyed out: RackMultipart201610313851719gh-066c35e1-b298-4e33-b0d1-630e0b3b5140-1009932244.png
Screenshots showing the tool bar's Sort: Last Comment Time working: RackMultipart2016103110257314c-c8ca2c2b-a424-4c13-9b74-bbecc854e706-155113881.pngRackMultipart20161031390341vvw-7370b113-1597-4786-9174-4454ff8af874-983729666.png
My option in menu View -> Sort -> Last Comment Time , is disabled, and if I use the option in toolbar, not work, for new comments and likes, in old collection work for old comments, but for new ones , don't work
Reading your previous posts, I think it would be best if you could provide a full-resolution screenshot of the entire LR window showing your issue (come back to the Web to post your reply, and upload a PNG, not JPEG, using the camera icon at the bottom of the reply window).
Those screenshots should be very helpful to Adobe.
Regarding the lack of the comment badge in the lower-left corner of the thumbnails, it looks like all badges are missing, including the badge that indicates a photo is in a collection. Verify that the option View > View Options > Thumbnail Badges is checked:
I give up, the reset not worked, then I decided lost all comments that I have, and use the option to delete all data in Lightroom Mobile, this work out.
I had the same issue and this was the first result from a Google search. For anyone else finding this you can still sort by last comment by going to: Library -> Review Comments
That looks like a bug to me. Instead of using the View menu, try using the sort order on the toolbar to select Last Comment Time, that should work correctly.