These home-router chips usually come up in a configuration appropriate for OEM WAN/LAN use and are “leaky” as a result until OpenWrt is running and re-configures them. They’re also generally reset on config change, but that is typically much less than a second so most never see the ill effects.
I don’t think there’s a good solution as I don’t think one can keep the phys powered down at boot. You might be able to do something with a custom boot loader, as the problem starts at power on. I typically use the WAN port as my VLAN trunk to help mitigate this somewhat.