tailscale using the routers public WAN IP (not the VPN IP) will be more reasonable.
We accidentally introduce this issue. 
I’ve seen a case where the s2s network overlays on OpenVPN client.
That’s a similar situation we’re facing.
The workaround to make tailscale traffic(port 41641 etc.) go through Non-VPN interface is to insert a IP rule, use the following command:
uci set network.policy_default_rt_vpn_ts=rule
uci set network.policy_default_rt_vpn_ts.lookup='main'
uci set network.policy_default_rt_vpn_ts.priority='1099'
uci set network.policy_default_rt_vpn_ts.mark='0x80000/0xc0000'
uci set network.policy_default_rt_vpn_ts.invert='0'
uci commit network
/etc/init.d/network reload