AFAIK it is not like the sheer moment the port goes down, for whatever reason, the MAC of the bridge changes, there are some events/changes that may trigger this change, depending on a number of factors.
The behaviour is known and the official Mikrotik recommendation is to set auto-mac=no and assign manually the MAC to the bridge:
http://forum.mikrotik.com/t/bridge-auto-mac-issue/162131/1
http://forum.mikrotik.com/t/bridge-auto-mac-issue/162131/1
Besides, it is what the default script running on most devices do at first configuration.
Like other Mikrotik “peculiarities” I personally believe that the best course of action is to avoid having a configuration that may - even in only a few, rare cases - create the issue, since it costs nothing or nearly nothing, hence I listed it as Rule #6:
http://forum.mikrotik.com/t/the-twelve-rules-of-mikrotik-club/182164/1