Hi
I have also experienced this issue in the past, unfortunately because you are now consuming the service through an API, none of the form rules are triggered. I assume you are running the DDR on form load, but since there is no user interaction, the onload event does not get triggered.
There are some workarounds you can use if it's just one single select you are using that does not use namespaces or reference other fields to populate the single select: Instead of using the DDR, you can pre-populate the values in the display properties for the single select, this should then validate correctly against the API.
Hopefully this approach works for you.
Regards,
Paul