Did you recreate your load test once the POST became implemented? Test Studio won't recognize the change in the old load test.
If you did re-record your load test, please send it to me for analysis.
Could it possibly be something to do with the control they used or some way they coded something?
I can see where one might think that but very unlikely. Remember a Test Studio load test works down at the low level HTTP request/response level. At this lever there is no UI, no controls. Only requests going out with parameters and response coming back from the web server.