I'm using a RB951G,and I grouped the lan1 and lan2 in a bridge.
I create two bridge filter rules:
/interface bridge filter
add action=drop chain=forward disabled=yes in-interface=lan1 out-interface=lan2
add action=drop chain=forward disabled=yes in-interface=lan2 out-interface=lan1

It works well before ros v6.19,but in v6.20 and v6.21rc,once I enable the rules and then disable them,the two rules seems always work.The only way to "really" disable them is reboot the router.

Hope you can deal with it.