Hello Tomas
Welcome to the Microsoft community!
I have read your description. Is the customer's environment mounting the virtual disk to the VM via ISCSI on Hyper-V Host? It is similar to the link below.
https://thesolving.com/virtualization/how-to-configure-a-pass-through-disk-with-hyper-v/
So VM did not use VHD files. Just transfer data to the Host via the ISCSI protocol.
Wish you all the best!
Best Regards,
Andrew