Same problem here, sometimes it works, sometimes not. Also figuered out that the problem occurs the most if you load into the same version of a project for the second time (Private Version; Playtest Version; Live Version). After loosing the progress once, the save point device seems to work more consistent.
Tested it with multible accounts.
I also figured out that i often get the ‘‘Network Connection lost’’ Error Screen in Fortnite after leaving my game and try to load back into it (Private; Playtest; Live Version). Everytime i get this error, all Sava Data is deleted too.