This happens to me also. It's very annoying.
My configuration is very diffent from yours, but perhaps my experience can shed some light on why you are seeing changing mac addresses on the bridge.
I use static WDS, which needs to share mac addresses of the radios on each side of the connection. Since the radios are under their respective bridges, the WDS configuration uses the mac of the bridge. If the bridge mac changes, WDS fails.
It appears to me that the bridge picks the mac address, from among its ports, that has the highest numerical value. If another port is added, the bridge's mac address might change.
This happened to me when I swapped out a radio card with one that had a higher mac address. This caused the bridge to start using a new mac address which the WDS on the other end of the link didn't recognize.
This will also occur if one of the ports under the bridge is a radio in AP mode. If
the radio has no clients connected, it will be marked as not-running and its mac address will not be used by the bridge, even if it is the highest mac address.
If a user connects to the access point, the radio will be marked as running, and its mac address will now be used by the bridge, if this radio had the highest address.
The result is that the mac address of the bridge keeps changing as the AP goes between active and idle.
As a work-around, I changed the mac address of one of the bridge port devices which is not an AP, making it the highest mac address. Now, the bridge always uses that mac address, regardless of the state of the AP.
Another work-around, which I haven't tried, is to disable the feature that marks the bridge port as disabled when there are no clients on an AP. I don't use that feature, but I think it's called "disable-running-check=yes" on the terminal interface. I can't find it on winbox.
I don't think the algorithm for selecting the bridge mac address is documented. Therefore, if you upgraded ROS, things might have changed. Or, if you changed any hardware that was under the bridge, the new HW might have a different mac address. If you don't have any access points under the bridge, try the trick of selecting a manual mac address for one of the devices that is high enough that it will always be picked.
Please post and let us know if this helped you.
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