So according to the man-page you cited it means the gap must not be greater than two minutes. But it's sometimes several hours and it lasts for hours even if you write again something. IMO it's simply a bug.
I would suggest creating a repeatable test case and filing an ITS then, since one generally does need to be able to validate existing contextCSNs across a cluster. ;)
--Quanah -- Quanah Gibson-Mount Product Architect Symas Corporation Packaged, certified, and supported LDAP solutions powered by OpenLDAP: <http://www.symas.com>