Exposing >4GB Graphics Card Memory to VM using DDA

Hello,

We have a Hyper-V Server 2016 with a PCIe graphics card with 8GB of memory exposed via DDA to a Windows 10 VM, but we can only see 4GB of graphics card RAM available from inside the VM. We used the "Set-VM -HighMemoryMappedIoSpace 8000MB" command and from the hypervisor we can see 8GB allocated, but it's not available from within the VM.

Is there an artificial limit within Hyper-V that is restricting DDA from exposing greater than 4GB of graphics memory? Or are we simply doing something wrong?

Thanks,

Trevor