I'm not sure what the mechanism is, but I discovered the issue by empirical testing a few months ago, and I'm still trying to sort it out and make the LAN visible to that VM. Granted, I haven't been able to put much time into it, but I keep ending up down among the weeds of iptables/nftables, with all the documentation either out of date or not intended for a host using netifrc for network management (or both). I'll probably have to ask on the Gentoo boards eventually.