Recently we faced a strange issue in one of our Actions. We were not getting output parameter’s value in the organization response.
Eventually, we realized it was because of the Stop Workflow step added. Removing it fixed the issue for us. We also tried by adding few other Steps like send email, status change, create record etc. after Assign Value step, however, in all those cases it properly returned the response.
Only in the case of Stop Workflow, we got the issue where the response was null for that output parameter.
Hope it helps..