Just experiencing the same issue and think I have worked out what caused it for me.
Duplicated a project with 75Gb of render files, checked copy render files in dialogue box.
Renamed project prior to completion of copying render files.
Started editing and rendering etc, restart app and lost all rendering.
Re-render files, move to event browser and back to event, renders ok, restart app, renders lost.
Checked Event folder on HDD (12TB Raid) and only 43GB of files in folder.
Started event duplication process again, (yes I deleted the previous event)
Duplicated event, with render files, let them all copy across, Background task manager confirmed complete.
Checked that full amount of render files in the folder, ok.
renamed event, make some changes and render
-quit and restart app, all ok!
Hope this helps and curious to know if this is what others may have done and is what is causing issues.
Tony
(crickey, anyone else finding this event/sequence/timeline/program/project terminolgy shift confusing?)