Was that default route even there in previous Zero images?
As of now, the Pi Zero (W) OpenWRT images seemed to have been designed with almost no network in mind (to me kind of unusual for an OpenWRT device. OpenWRT = in my eyes: you want to use it as a network device. And since it has USB and HAT support and no 4MB flash limit, this default config decision seems unexpectedly restricted)
To me this makes the Zero particularly difficult to setup compared to other OpenWRT devices: no built in RJ45, no Wifi on by default (which is usual, but additionally difficult if you cant use LAN either), no USB LAN dongles supported by default, all reasonable OpenWRT default network config content wiped. You only have the plain console.
Currently I create a Zero custom image right away, to prefit it with USB LAN dongle drivers + customized UCI init script to get back the well-known OpenWRT network config file defaults. But once you made it past that steep initialization curve, it is a super nice 1 Watt sensor device + mini server/network device, from my perspective much better than with Raspberry OS.