Very interesting. 
Can you post the text export of the relevant settings for your VLAN 312, both the “bridge” and the “switch” settings, so that I can maybe understand how the “normal” bridge ones are translated into the switch ones?
I am trying to help a user with his configuration, and now everything seemingly works with bridge, and wanted to see if switch would bring any advantage, JFYI:
http://forum.mikrotik.com/t/dhcp-relay-on-one-interface-only/182004/1