It could be that dropbear is simply not listening on IPv6 (yet). I use IPv6 ssh on the lan all the time, but I have to restart dropbear every time I reboot the router, as there is some lingering timing issue with its startup. (There were a bunch of commits for dropbear just today, I'm hoping the last one resolves this: https://git.openwrt.org/?p=openwrt/openwrt.git;a=commitdiff;h=3f96246e97215c4c76ca407a8bca8f3f5de32e1c)
In LuCI, try restarting dropbear in System -> Startup, or from cli /etc/init.d/dropbear restart and see if that helps.