Misconfiguration of bridge/VLAN/L3 offloading is the likely cause, too many Linux DSA expert users in this forum:
http://forum.mikrotik.com/t/mdns-repeater-feature/148334/322
Some common examples:
https://help.mikrotik.com/docs/display/ROS/Layer2+misconfiguration
If the above don’t apply to you, then it’s likely the lack of RFC4638, read this:
http://forum.mikrotik.com/t/how-to-edge-router-and-bng-optimization-for-isps/150007/1