Hyper-V only supports UEFI or GPT partitions for Gen 2 therefore this is a expected behavior. BMR for UEFI Partitions are only supported for Windows 2012+ using Hyper-V Gen 2.
Workaround
1. Attach VHD to the system
Go into Disk Management
Click on Action
Click on Attach VHD
Click on Browse and select the VHD file
2. Convert GPT Disk to MBR Disk
Use application AOMEI Partition Assistant
Select the Disk on the GUI
Right click on it and select the option Convert to MBR Disk
3. De-attach VHD from the system
Go into Disk Management
Right click on the attached VHD
Select De-Attach VHD
4. Configure as active the partition
Attach new VHD into VM
Boot VM into Windows installation disk
Press Shift+F10 when the first window appears to open a CMD prompt
Type DISKPART
List Disk
Select Disk X (Select disk that was converted)
List Partition
Select Partition X (Select OS partition)
Active
Exit
Reboot VM
5. Fix Boot
Boot VM into Windows installation disk
Press Shift+F10 when the first window appears to open a CMD prompt
Type Bootrec /fixboot
Type Bootrec /fixmbr
Type Bootrec /rebuildbcd
Exit
Reboot VM
Boot VM into Windows installation disk
Click Next
Click Repair (it should recognize a Windows installation)
Press Shift+F10
Type CD Recovery
Type StartRep
Click Finish
Reboot the machine and let it boot into Windows, it should boot fine now.
