I think it's the "first to connect" logic between IPv4 and IPv6. Saw an article recently that said when the address being connected to has both addresses, hosts doesn't work unless you map both there, otherwise the unmapped address will connect.
That doesn't explain this happening on systems with 2009 Windows 7-era hardware which is only IPv4 capable, like one of my systems - I've noticed HOSTs being ignored for many, many years.
It's not very practical though. Better to block the address with little snitch or a hosts file