Hello Steve,
you may have stumbled upon a limitation of the native REST functionality. Haven't tested myself to confirm this...
In the meanwhile you might want to integrate "by hand", that is, do the requests yourself.
Take a look at these components in the OutSystems forge:
https://www.outsystems.com/forge/component/698/how-to-rest-made-easy/
https://www.outsystems.com/forge/component/422/restwebservices/
you may have stumbled upon a limitation of the native REST functionality. Haven't tested myself to confirm this...
In the meanwhile you might want to integrate "by hand", that is, do the requests yourself.
Take a look at these components in the OutSystems forge:
https://www.outsystems.com/forge/component/698/how-to-rest-made-easy/
https://www.outsystems.com/forge/component/422/restwebservices/