it never really bothered me having to turn it off because it's a one off thing per project so I never looked to see if there was a way to turn it off by default

I don't know why it's on by default either... i'm sure there's some reasoning behind it... i just don't know what and i don't know what to speculate either.
but i'd assume something to do with something... yeah... real perceptive there huh?

also if you changed the thickness of border edges you'll see the issue even if you don't have two sided lighting turned on

go to Windows > Settings/Preferences and in Polygons there should be an option to highlight border edges. tick that and change the thickness to 4.0 and things will be way better

you can also tick Soft/Hard in the edges as well. I find having these on a lot more informative