I bumped into this a couple of weeks ago, I too think it was after a Qubes kernel update. I didn’t investigate until today, but what I found out was that no IP address is set on vif interfaces when attached, it seems like /usr/lib/qubes/setup-ip is not triggered (I put ‘thouch /tmp/thing’ right under the hash bang and no such file was created a new VM and vif was attached).
With ‘dmesg -w’ I saw UDEV/KERNEL lines with add+online and offline+remove, but none with bind or unbind.
With ‘udevadm info --attribute-walk /sys/class/net/vifXX.0’ I saw ATTRS(state)==“InitWait” where it should be “Connected”
Manually setting the same IP on the attached vifXX.0 as on the other interfaces, ‘ip address add dev vifXX.0 10.137.0.17/32’, enables the Whonix gateway to route traffic from connected VM through Tor.
I have not yet tried with a regular Debian (Stretch) template as netvm to see if the problem is the same. There are no issues with a netvm with Fedora 29 or 30 as template.