Dynamic pick list not working

Quick question:

I've created a dynamic pick-list that updates based on the value of a another field (field_A). The filter criteria for the pick-list was entered during form creation.

My problem: the pick-list updates correctly if I manually enter a value into field_A, but not if field_A is updated via a script. Shouldn't it behave the same in either case?.

A more detailed description is in an unanswered earlier post.

Thanks for you help.