course instances changing each other

Hi

I have a single course with three course instances in the schema markup (all on one page). When I edit the start dates and end dates for one of the instances it is also altering a field in another instance.

From what I can see changing the end date for instance 1 also alters the start date for instance 2. If I edit the start date for instance 2 it then seems to alter the end date for instance 1 etc.

Is this a known problem and is there a fix?

Many thanks

A