Who Can Read buttons on Edit Space Info page seem to work inconsistently

(Bug, Investigate , Priority: Critical, Test Status: No automated tests yet , Reported By Justin du Coeur, )
Security
Summary: When I press the "Everybody" or "Members Only" buttons on Edit Space, I frequently wind up in an inconsistent state.
The thing is, these buttons are setting two values: on the Space itself and on the Instances. It seems to be consistently saving at least one, but frequently not both.
The network traffic looks right, so this smells worryingly like some sort of server-side race condition, caused by the two simultaneous saves. I don't see where the problem could be, though -- they should both be resulting in idempotent ChangeProps messages, aren't they?