This seems to be a relatively recent change, as it wasn't happening on my previous build.
It appears that when a wireguard interface defined in
/etc/config/network comes up, it is adding an explicit route to the tunnel endpoint via one of my two internet interfaces. This breaks things badly, as I'm using mwan3 - certain things to this endpoint go out one interface, while latency sensitive traffic goes out the other interface. With the explicit route added, none of the mwan3 routing works.
Does anyone know where the code is that adds this route?