Is there any post activity configured to the local flow action?
if yes, could you please delete & test the behavior
Could you also please try disabling the option 'Look for an assignment to perform' for the local flow action & configure the option 'If an assignment is not being performed' to Show Harness --> Confirm
Configuring custom buttons with onClick event to launch harness would render harness even if errors present on screen. (validations fail in this use case as you mentioned)
so could you please try enabling the button only when there are no errors on Primary page allowing the user to hit submit.
Please share your thoughts/observations, Thank you!
it seems that you are trying to execute a local flow action against a case which has reached the end of its flow execution, and it is not usual. Can you explain a bit more about why you have to configure the case in such away?