"Temporarily disabled from" (DeactivationStart@Person) and "Temporarily disabled until" (DeactivationEnd@Person) are set and the appropriate automation process "Lock user accounts from dismissed employees" is activated (schedule "Lock accounts of employees that have left the company").
If the Employee reaches the start date (EntryDate@Person), first the "Disabled permanently" (IsInActive@Person) flag is set to True, then "Temporarily Disabled" (IsTemporaryDeactivated@Person) is set to True as well. This results in the date fields, DeactivationStart@Person and DeactivationEnd@Person having null values, thus making it impossible to reactivate the Employee.
This is a product defect (25013).
This will be fixed on a future release of the product (SP2). If you require this immediately corrected, please contact Support for a hotfix referencing the defect ID 25013.