Out-of-Office status is synchronized by MAgE only once during the initial synchronization. Further changes of this status are not synchronized.
This is acknowledged as defect PT127993043
This issue has been registered as PT127993043 and fixed in Public Update 20161013EX for MMEX 8.12 and in Public Update 20161028EX for MMEX 8.13.
To resolve the issue, please install the latest available update for the appropriate MMEX version and resynchronize the problem mailbox with Migration Agent for Exchange (MAgE).