Understanding USB block device control: mounting to sys-usb makes the block device disappear from qvm-block output

I am trying to understand USB block device (storage USB) handling in qubes.

I have a simple USB flash disk which I plug into my QubesOS. I can see the plugged in block device in the output of the qvm-block command.

However, when I open the Terminal in the sys-usb qube, and mount one of the device partitions of this USB, with the following command:

$ udisksctl mount -b /dev/sda1

when I do this, the mounted partition disappears from the output of qvm-block command in dom0. Why does it disappear?