When adding a new part to a UI action, the general list of actions should come first, the possible parameter should be shown when you move the mouse on Parameter Mapping.
The reason is that so far this functionality was made for model elements which were mainly parameter-mapping only (workflow step parameters). In this case it makes no sense to offer “Screen Condition” for mapping a parameter anyway.
So this is not really a bug, it works this way in general, although it is not optimal.
The solution would be to somehow decide which model sub-elements are parameter-specific and which are not. Also it is not possible to wait until the person clicks on “Parameter Mappings” as this part is not there until you create at least one parameter mapping. So the offering when clicking on the parent element must offer all the sub-elements.