I've completely forgotten about 'default' network which I've never used.

But what is pretty confusing to me are the circumstances under which "it" happens, namely...
There are no guests using 'default' network - there is DHCP server on "physical" network (same host as libvirt)
I can confirm dnsmasq is not started, is not running, not on boot nor later, then...
"main" DHCP server goes down for whatever reason and...
this happens, libvirt/dnsmasq slaps itself onto main/all network & actual DHCP is unable to come back - for obvious reason - when it does.

Why would libvirt/dnsmasq (mis)behave like that?
I've now disabled 'default' net's autostart but... jee.

thanks, L.