@MaxonL16591286 We had a similar requirement(in 8.6) where in we are displaying a list using Menu Rule, and onchange we are running a DT and based on a logic we are setting .IsMessage=true/false. If it is true only we should display the modal dialog. But modal dialog was always showing even if it is false also. So as a work around in the modal dialog section, we added 2 different paragraph rule for displaying positive/negative scenario message(and those 2 were displayed on basis of .IsMessage=true/false). Even though it is an extra click for the end user when IsMessage=false they were fine with that as the message we displayed was meaningful and contextual.

@MaxonL16591286  the support article was written to inform users of the correct design methodology.  The issue was never recognised as a product defect and hence the behaviour did not change between Pega versions.

Also note that a When rule on Local Action would consider property values only from DOM and not from clipboard.   In order to make a scenario work where a when rule is set before calling an activity ,make below local changes :

1) create a "Hidden text" field, say A in the  section where button is configured. 2) Call 'Set Value' action before "Local Action"  to set the property- A (created in step 1)  with activity output value . 3) Check for property - 'A'(defined in step1) on Local action.

Please follow the advice given by @Anoop Krishna .