-localhost is supposed to only bind the local interface only. It appears that when testing the availability of the display number the function "displayNumFree" creates an unreferenced TcpListener on 6000+n where n is the available display number being tested. The port is open on all interfaces. No connections are accepted on the port. Maybe this could check for -localhost and/or just bind local interface in all cases?
On further reading, this appears to happen in unix/xserver/hw/vnc/xvnc.cc line 355 in displayNumFree()
Please report this upstream.
https://github.com/TigerVNC/tigervnc/issues/115 Thanks.