That’s it! Somehow I’ve missed your comment, but also now it happened to me and I’ve found out the same.
vif* interfaces is responsibility of
/etc/xen/scripts/vif-route-qubes script called by
xendriverdomain service, not udev. It fails, with logs in
/var/log/xen/xen-hotplug.log. I see a single error there:
RTNETLINK answers: Permission denied
set -x at the beginning of the script, I see it’s on a try to set IPv6 address. Probably because IPv6 is disabled in Whonix.
Indeed I have enabled IPv6 in my system globally (https://www.qubes-os.org/doc/networking/#ipv6). The same page explains how to disable it for particular VM (and others connected to it). So, I’ve executed
qvm-features sys-whonix ipv6 '', restarted sys-whonix and now it works.