The problem of the PPPoE connection on the bridge interface dropping when wireless clients connect and disconnect turned out to be caused by the bridge MAC address changing. The solution was to set admin-mac to ether1’s mac address and auto-mac=no for the bridge interface, similar to this thread:
http://forum.mikrotik.com/t/how-to-prevent-bridge-mac-address-changing/36590/1
Thanks to Sergejs of MikroTik support for this! 