Please keep in mind of the following when backing up Hyper-V replica VM.
1) Since transmission of backup data of Hyper-V replica VM is not destinated to a fix host, at the timing of backup task exection, you have no way of telling the backup data will be saved for primary or replica VM.
As a result, if you select the latest backup image to restore from replica VM, the backup data may be deficient.
2) When the changed data transmission and backup task execution concurrently take place, the replication or the backup task fails.
When the replication task fails, you have to manually run the replication task again.
Due to the above reasons, you are recommended to install ActiveImage Protector on the virtual machine on Hyper-V host when backing up the Hyper-V replica VM.
In this case you need to purchase the Virtual Edition license as many as the host machines.