I tried FreeBSD 11-CURRENT VM on Windows 10 host and found the VM network could easily stop working about 5 minutes after the VM finished bootup.
After some debugging, it seems the host virtual switch has some issue for wireless NIC. It seems if we change the default ARP expiration time from the default 1200 seconds to 60s, the issue will disappear.
Details about the workaround:
Add "net.link.ether.inet.max_age=60" into /etc/sysctl.conf and reboot the VM.
Or, "sysctl net.link.ether.inet.max_age=60" and "arp -d -a".
The real cause of the issue should exist in the Hyper-V host side and it's unlikely to get this fixed in the near future considering the effort needed and its priority, as far as I can tell.
And since we have an effective workaround (see the original description when I reported the bug), let me close the bug -- I chose "Works As Intended" because I couldn't find a better type like "WON'T FIX". Feel free to correct this, if you know how to do it.