When a user enters time into a timesheet, the revision history displays all the revisions correctly using the API.   If a user deletes that entry all revision history is deleted for that date in the API instead of displaying that the hours have been changed to a null.  If they enter a 0 the revisions remain intact.  

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Release: AGUE5099000-SAAS-Agile Central-Unlimited Edition SaaS-50 User Pack
Component:

This is working as designed.  Essentially, when a time entry is deleted by the user, it is a real delete for the hours on that date and it deletes the revision history too.  

Workaround is to have users change the time to a zero instead of deleting hours.