We've been having issues with Tasks Lists in SharePoint 2013 Enterprise On Prem.
The "Assigned To" user for some tasks on any given task list seem to be randomly clearing out for some users. What's odd is that it's usually only one users tasks affected, but it happens to ALL tasks assigned to that user.
Sometimes it's the user who created the list and assigned all the tasks, other times it's another user. But whenever it happens all the tasks for that user are cleared, regardless of anything else (due date, progress, etc).
Usually after re-assigning the tasks it works fine but sometimes it clears it again.
One other thing to note is that I recently setup the relationship between our Exchange 2013 and SharePoint 2013 server so tasks can be synced between Outlook and SharePoint. This feature has been shaky at best, sometimes the tasks lists in SharePoint will sync, other times they won't.