The dynamic disk is the VHD dynamically expanding or You are using Dynamic Disk inside the VM?
If you are using Dynamic Disk inside VM, so it may be caused BE to run a full backup instead. I am also using Hyper-V VHD dynamically expanding without this warning.
Dynamic Disks (http://technet.microsoft.com/en-us/library/cc757696(WS.10).aspx) – Virtual machines using Dynamic Disks can be
protected using the Agent for VMware and Hyper-V. However, online backups of these virtual machines are unsupported. Granular
recovery of Dynamic Disk volume backups captured through the Agent for VMware and Hyper-V is also unsupported.
Refer to :