Why do cascading parameters based on a scripted data set not work correctly?

In RTC 6.0.3 (but it also occurs in RTC 5.0.2) I have a two-level cascading parameter based on scripted data set. The top-level is required and single-value. The second level is optional and single-value. I have two issues:

Firstly, the "Sort By" works in reverse when running as an installed report (but works OK in the Report Designer)

Secondly, if I select a level-1 value and then a level-2 value and then change the level-1 value, the level-2 value stays the same and is usually invalid for the new level-1 value. It works fine if the level-2 parameter is multi-value

What might I be doing wrong? I have a small rptdesign file but can't see how to attach it