The logic is a following:
in the user permissions you have "Change manager" permission and "View other Admins' action items ".
A user without these permissions can see an update only his assigned actions.
A SysAid admin with "SysAid administrator" permission can update anyones actions.
In the 6.5 there is a new field (FR 3712) that contains the last update info, so you can clearly see who updated the action item and when,
In a normal environment - where not all admins are "SysAid Administrators" you shouldn't experience the scenario you are describing. try to test it with a normal Admin and let me know.
And please add the 2 new fields to your views (Reopn info and last update info)