Yes and no.
In the sense that the way Mikrotik sets the MAC of the bridge is during the running of the default configuration script, and it does so by assigning to the bridge (and setting auto-mac=no) the MAC of the first interface it finds among the ports in bridge.
So, since:
- the default configuration puts ether1 self standing and ether2-5 into a bridge
- the interfaces are added in the order ether2, 3, 4 and 5
the net effect is that the automatically assigned MAC of the bridge is the same of that of ether2, and the reason why it is “forced” is to avoid that with auto-mac=yes it could change when the user removes/adds or however fiddles with ports on the bridge, and that is the main reason I believe why assigning a MAC and setting auto-mac=no is recommended for the bridge.
The risk of a duplication (only possible when/if you take ether2 out of the bridge) is lower than having the bridge MAC changed inadvertently, though there are diverging opinions on whether forcing manually on the bridge yet another MAC (different from all the ones given to the ether and other “self-standing” interfaces) is a good idea or in practice an unneeded precaution, JFYI:
http://forum.mikrotik.com/t/doubts-on-mac-address-on-bridge-uniqueness-laa-etc/182230/1
There is not written anywhere that the bridge MAC must be one of the ether ports in it, it could be a completely different one.