Ok, so I understand this now. Summarizing for the record.
multicast_to_unicast is a setting on the bridge interface, not on the specific interfaces such as lan1/2 or wlan0. This is confusing because if this setting is not set on the bridge interface then it defaults to 0 for wired and to 1 for wireless.
To disable this on all interfaces of a specific bridge run:
uci set network.lan.multicast_to_unicast="0"
uci commit network
/etc/init.d/network restart