The bit is being set for a reason, it means that the MAC address was locally generated and may not be globally unique.
See the diagram in the Mac Address wiki page for more details.
Since Windows uses a closed source model, it will probably be impossible to change this behavior.
Also, I am not clear on this part but because documentation is not available there might not be a better explanation available. The way Windows creates bridges is that it creates a virtual adapter associated with the real adapter and sets the mac id as you described. It then proceeds to change its routing tables associated with that adapter so that it can form a bridge.
In short, no there doesn't seem to be any documented way to prevent that behavior.