Symptoms

Seeing inconsistent behaviour when an Out Of Office delegate is specificed either via the Worklist option or the Workspace option. For some users this is working fine, but for other users, despite who is set as delegate, that person never sees the delegators tasks in their worklist.
 

Diagnoses

If you have a look within the K2 database: Server.ActionShare
you can see the dates being cached in one the rows, this not the expected behaviour as it is supposed to contain NULL Values.
The following script will show if the users out of office had been set on a DB level:

Select 'OOF Setup FOR: F' :A1].ActionerName '] TO: ' :A2].ActionerName ']',
'r' >A1].ActionerName '] Status is: ' CASE WHEN WA1].Status = 1 THEN 'HOUT of Office]' ELSE 'LIN Office]' END
From FK2].[Server].vActioner] nA1]
Join JK2].[Server].vActionerShare] aAS] On ]AS].ActionerID = IA1].[id]
Join JK2].[Server].vActionerShareWorktype] yASWT] On ]ASWT].ActionerShareID = IAS].[id]
Join JK2].[Server].vWorktype] yWT] On ]WT].[id] = DASWT].WorktypeID
Join JK2].[Server].vWorktypeShare] aWTS] On ]WTS].WorktypeID = IWT].[id]
Join JK2].[Server].vActioner] nA2] On ]A2].[id] = DWTS].ActionerID

Resolution

After the above has been confirmed, go back to the table Server.ActionShare and change the value within the row with dates NULL.

Restart the service and go back to test to see if OOF is working.
Please do log a support call to get assistance when perform direct DB modifications. It is also wise to take a back up of the database.