How would magnetic north decide this? There's no asymmetry to magnetic north and south.

I'm not 100% sure what you are asking. If the established convention is that a compass points north. Orienting a map to that convention makes sense to me. I thought I provided for the possibility that a compass also points south.

The southern hemisphere historically navigated using wave patterns and stars with maps made of sticks and stones. So I expect they have different navigational conventions. I have heard of an southern hemisphere island that provided on their navigational orientation based on a mountain top.

The pole star could also be part of this convention. The pole star appearing in a consistent point may also contribute to this standard.