Hi, Richard. This looks like a finger pointing on DELL side to me. I seriously doubt the issue is in Windows Server 2012 R2, because we have huge amount of customers using Hardware VSS assisted backup with other storage devices with great success. Including DELL's own storage (I have not heard the same complaints from DELL Compellent users). For example, here is a white paper from Hyper-V MVP
who has done a really hard core testing of Veeam on his Compellents.
If this is a "known issue" with Windows Server, can DELL perhaps provide a link to the Microsoft support KB article? One thing Microsoft is really good at, is documenting known issues. And this will give me something to go to Microsoft Hyper-V team for clarification.
One thing we normally recommend is to disable ODX - not sure this is related to your issue, but it certainly helps to address a bunch of other issues with Hyper-V backups.