I only have an RT-AC68U left over and can use/test media bridge with it (before I used an RP-AC68U but it broke, that's why I ran into this issue now lately with 386.11). But telling dnsmasq that it shouldn't give IPs (no-dhcp-interface=br0) fixed it for me. Obviously this statement seems missing in 386 as in 388 it's there (see RMerlin's example).