How many milliseconds does it take (on average) for the request to complete in Postman? I have seen this behavior before when the consuming side has a very strict (low) timeout setting on their end.
In other words; the 200 OK response might not be returned fast enough for the other side, causing a timeout and (endless) retry of the message.
In order to verify if this is the issue, check the timeout on the calling side and verify it against postman results. Another solution is to temporarily change the published microflow so that the only thing it does is return the 200 response code (and maybe a hard-coded response body, just something that returns a response really fast).