Question: Are newly appointed government employees already entitled to leave benefits during their probationary period?
Yes. All government employees, except those covered by special laws (e.g. justices, judges, those in the foreign service), are entitled to leave benefits immediately upon assumption to office because the Omnibus Rules on Leave do not make any distinctions. Moreover, leave credits are earned based on actual service rendered, using a given table of computation which you can check here. And, actual service, pursuant to Sec. 28 of the ORL, commences from the appointment of the official or employee.
Even more precise is Sec. 44 of the Omnibus Rules on Leave, which reads:
An employee still on probation may already avail of whatever leave credits he has earned during said period. Accordingly, any leave of absence without pay incurred during the period of probation shall extend the completion thereof for the same number of days of such absence.
The first six months of service following a permanent appointment shall be probationary in nature pursuant to Sec. 4, Rule III of the CSC Revised Omnibus Rules on Appointments and Other Personnel Actions.