Virtualbox guest files keeps corrupting (differencing child hard disks)

I’ve Windows 10 guest on BTRFS partition. I’m periodically taking BTFRS snapshots and backing it up.

My BTRFS snapshot script checks whether there is an inaccessible machine or not before taking snapshot:

while read -r vm; do
    echo "Inaccessible VM found: $vm"
done <<< $(VBoxManage list vms | grep inaccessible)

So, I’m pretty sure that there is no Inaccessible VMs before I make a successful backup.

When there is an inaccessible VM found, the backup system aborts and displays an error message, so I could immediately take care of the inaccessible VM.

Lately the following error occurred so many times:

This VM can not be started: xyz.vdi cannot be directly attached to the virtual machine 'Foo' because it has 1 differencing child hard disks.

I try to restore the previous files, but it keeps displaying this message. I tried to restore the VM files from the other disk, but it still displays this error message.

The VM was stopped for days, so, I don’t think VirtualBox has anything to do with that, but I’m not sure.

Why this error occurs?
Why restoring previous VM files does not resolve this issue?