Hello @Chris Hibberd ,
this is an interesting observation.
I was able to enter both DeviceIDs in an IoTHub:
So, this IoTHub does not seem to be the issue.
In the DPS, I can register that DeviceID, only the RegistrationID must be simplified:
(I use individual enrollment, symmetric key)
So the registration works by hand:
I tried this with a code example from the Github repo Python Azure IoT Device SDK:
So enrollment and assignment works with symmetric keys using a Python SDK example.
Please check if this works for you too. If so, try the X509 sample.