new revisions only being created for user/1

I have 'create new revisions' set for a #node and Access Control perm 'Edit #node content' checked for authenticated users.
I fiddled with the 'View Revisions' and Revert Revisions' perms but this made no difference.

Every time I edit the #node as user/1, a new Revision is created for each submit.
Every time I edit the #node as user/2 or user/3, no new Revision is created, but the Current Revision is overwritten.

The problem is occuring at the level of the database table node_revisions. It appears that for any user other than user/1, the new vid is not being created, and the uid, title, body, teaser, etc are being edited.

So I guess it's a Query bug ... ?