Hi srhh,
Was having the same discussion on this thread: Hotplug CPU windows server 2019
Looks like the issue has been resolved with this patch: https://www.catalog.update.microsoft.com/ScopedViewInline.aspx?updateid=a7738ed8-a5f9-41e4-b968-af381f7c8988
I applied the 2019-02 Cumulative Update for Windows Server 2019 update and after that I can happily hot-plug addtional vCPU without issues. There is nothing that I can see in the release notes about this resolving this issue however let us know if this works for you.
Kind regards.