Edit 2:
Should the “Action node failed” trigger at all? It does not fully make sense to me as the action has run its course 100% as intended. As such the action node has thrown its error successfully and that error should be exposed. Same as “end excecution”.
I have been able to recreate a similar issue in my environment. It looks like something fails in the Throw Exception action node, which then triggers the “Action node failed” event, which overrides the 409 error with a 400 error.
Would you be able to share your exact logic before throwing the exception, so I can recreate the issue using your specific setup, and register this internally as a bug?