This can happen if you force-quit Pyblish QML while having hosts still running.
The problem is that the hosts still broadcast "heartbeats" to Pyblish QML at even intervals, and as soon as you quit it and start it again, it will record the port of the heartbeating client when it comes in. If then Nuke heartbeats before Houdini that one particular time, their port numbers will get mixed up.
Should resolve itself shutting down all hosts, and then opening them back up. Force quitting of Pyblish QML generally shouldn't happen, but it can be supported, it just isn't yet. Safest thing to do is the above.
You can tell which port a host is listening on like this.
This same port number should then be reflected in the Terminal of Pyblish QML when running it from this host.