I did happen to log a defect with Apple, but they closed it citing it was a change in behaviour. I don't see this behaviour change reflected in any iOS MDM documentation.
This is the actual response:
This is a follow-up to Bug ID# 38757618 - AvailableOSUpdates command fails in iOS 11.3 beta
Engineering has provided the following feedback regarding this issue:
This is correct behavior as of iOS 11.3 betas. When no updates are available we return 2213, “No update available.”. This is a behavior change since pre-iOS 11.3, when we just returned an empty array.
The description of the underlying error from software update services could be better. Error code 3 is indeed “update not found”, but the description “The operation couldn’t be completed.” could be misleading.
We consider this issue closed. If you have any questions or concern regarding this issue, please update your report directly (https://bugreport.apple.com).
Thank you for taking the time to notify us of this issue.
Best Regards,
Developer Bug Reporting Team
Apple Worldwide Developer Relations