A fun one... for all those times I've seen people recommend 'Disable #IPv6' as a universal band-aid to all network problems.
I *finally* had a problem where 'Disable IPv4' actually turned out to be the solution.
Turning this off seems to fix all the issues I've been complaining about on my timeline for a while Re: IPv6 and VMs in macOS. If I turn off v4 completely, all the issues seem to go away.