This seems to be a new "feature" in SQL Server 2008. To reproduce
Create a report from an OLAP cube and use one paramter.
Edit the MDX for the parameter (this is a common requirement to restrict member list etc.)
Open the main dataset for edit and save. At this point RS will regenerate the parameter datasets. This can be quite annoying if some effort has gone into customising the MDX.
My workaround is to maintain a notepad (one for each report) with a list of MDX expressions that can be cut and pasted into the datasets. But there must be an easier way.
Also note, the problem is often cascading with @parm4 using @parm3 using @parm2 using @parm1. So that when the main dataset is changed, all parameters need to be rewritten in the correct sequence.
I would be interested if anyone else has come across this issue and got a better solution.