To understand the issue, I captured the DVD and OS Disk configuration on Hyper-V manager. From the picture, we can see there are 2 IDE Controllers and each IDE Controller has 2 locations. Changing the controller for DVD and OS Disk will cause DVD missing on VM side or system fail to boot.
Created attachment 175174 [details]
exchange the IDE controller of OS disk and CD/DVD
The patch is under review: https://reviews.freebsd.org/D7693, which can also fix https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=212721