Huh.. You have a point there, I never considered that the revealing part being square could be an abnormality. Well, I never noticed it, to be honest.
Well sure I know it's not really an issue and I fully agree there. I just didn't know if it was intended or not, untill now.
As for the parts remaining revealed, or in this case, not.. I've also noticed that when I walk around and re-visit the parts I've explored before and open the map, some traces of where I walked could still be seen on the map. However they are kind of like a broken, chisled off version of a trail. Again, not really an issue because you're right, the waypoint system is more than enough to keep track as to what is where, and therefor find your way around.
I indulged in this mainly because I noticed that the person I followed a few videos of, on Vintage Story, had his map opened and closed several times and his radius was insanely huge on the map. It was also, as you so keenly observed, a circle instead of a square like mine. But his, for whatever reason, remained showing explored regions. Untill he logged out and back in, at which point it's reset. It's just a curiousity.
Huh.. I didn't consider the chunk loading option could affect how much of the map is revealed. I'll test it out and see if that changed anything.
Edit: You're right, it does affect that. Changing the view distance from 45 to 128 changed the revealed map area to a bigger size and changed the shape to a circle. I'm feeling quite stupid for not thinkig of that, but happy nonetheless for the fact that you mentioned it. Thank you!