Old thread, but I have this same issue about the same timeframe as these users. The strange thing is I was able to fix this problem (!) but I neglected to document it well. I have been looking at my history and trying to find what I did to fix it, as the problem started again for me.. What caused it to happen again was a windows 10 upgrade. If memory serves, I believe it is some reg entry that gets reverted that causes the problem.

Here are two videos that show the problem, since most folks don't really understand what is happening. what sucks is that the 'flickering' cause other network connections to drop/reestablish so it affects the host system as well as VMs.
The First video link about twenty seconds in really highlights why it is a problem.



Untitled on Vimeo
and from last year:
Hyper-V-DefaultNetwork-intermittant install-uninstall on Vimeo

if I don't get a reply, I will post a new thread with this content.

edit:
I should also add that I figured out that this is the service when running that creates the vDefault switch and removes it over and over. Further, when I was able to get the network to STICK momentarily, i noticed that the send count went up while the receive count stayed at ZERO. perhaps that is why it gets removed?