For me it looks like that avahi-daemon falls into an infinite loop sending out certain messages repeatedly (there are several 100 messages per second announcing the available CUPS queues) and does not listen any more to requests (loop does not go through the code for listening). Therefore avahi-discover cannot connect and also CUPS is not able to unregister its printers on shutdown. This leaves avahi-daemon continue to spin and announce CUPS queues.
For me it looks like that avahi-daemon falls into an infinite loop sending out certain messages repeatedly (there are several 100 messages per second announcing the available CUPS queues) and does not listen any more to requests (loop does not go through the code for listening). Therefore avahi-discover cannot connect and also CUPS is not able to unregister its printers on shutdown. This leaves avahi-daemon continue to spin and announce CUPS queues.