Custom data in group vs item level
I haven't specifically done any tests on Windows Classics platform for the MobiControl versions you mentioned. However, based on our previous tests on other device platforms for tens of different MobiControl versions, custom data mechanism should work on both per-device or group level.
Have you performed any tests on other device platforms (i.e. Android, iOS, Windows Unified, etc.) on your servers? If the results are consistently negative, I believe you should open an official support case with Soti Support team via email to email@example.com.
Thanks for the comment :) I have not tested with Android, yet, will test further though.
** Tested now, and for Android it works as expected.