If you want to install FreeBSD on Hyper-V for generation 2 VM, you will fail to boot because CD/DVD device does not work.
The root cause is FreeBSD storvsc driver does not handle a status flag propertly, as a result, the /dev/da0 or /dev/cd0 was detached finally.
See the fix https://reviews.freebsd.org/D10212
It was fixed on head r316519.
Normally wait tell the MFC is done and then the bug should close for you if the proper PR was refered to in the commits. I think this bug was held open waiting for your 3 day MFC timeout.
Reopen it until 3 days after MFC.