Can you please test with smaller packets as well? Quite a while ago, I encountered similar issue, where VLAN-tagged packets were not passing through the bridge in CHR but everything worked fine when I bound VLAN to an ethernet interface. All details here: http://forum.mikrotik.com/t/chr-does-not-transmit-frames-with-vlan-tags-from-bridge/127278/1
If it does not work for you even with smaller packets, it is a proof the proxmox is suffering from the same issue as hyperV where I encountered this bug and it is not about size, but about VLAN and bridge… Since then, I switched CHR to an ESXi and it works fine now.
Just to make sure, I tested your situation with my live CHR on ESXi (sorry, no lab equipment ATM) and I can easily send large (8500) packets through VLAN which is assigned to the bridge.
So it is definitely NOT global bug.