Updated 12/19/19 - The fix for this issue has been released with the 8C (Third week of August 2019) Windows Update, which is the 3rd week of August (See Windows 10 update servicing cadence).
Note: Any device that has 1809, but doesn’t have the August update will have this issue.
When enrolling devices through Autopilot, you can create an Autopilot deployment profile and configure the “Apply device name template” setting to create a device name for Windows 10 1809 and higher devices at initial startup.
We’ve discovered a bug in this setting with Windows 10 1809 where using a prefix with the %RAND:x% macro does not work to apply a device name, if the total of the prefix and macro adds up to 15 or more characters.
As a workaround you should ensure that you enter 14 characters or less overall when using device name templates for your Autopilot device enrollment. We'll update this post when a fix is in!
Blog post updates:
12/19/19 with an update that this known issue is now resolved.