It’s a limitation of the VHD format, VHDX would need adding, or if you can use ReFS for the backup store (I can’t in my setup) you might be able to use raw image format instead, some discussion here
It’s a limitation of the VHD format, VHDX would need adding, or if you can use ReFS for the backup store (I can’t in my setup) you might be able to use raw image format instead, some discussion here