Since the 10.94 update there seems to be a way to explore areas more fully than previously possible.

In areas where it’s possible to walk along the edges diagonally, doing so now reveals an extra tile on the map compared to just walking with the arrow keys.

As an example, here’s a small cropped area from the Deeper Banuta map. This is what you’d get after walking along the slanting edges perfectly using the arrow keys:

And this is what you get after walking diagonally along the slanting edges:

Note that there are fewer unexplored (black) pixels in the latter image. Here’s a before/after comparison of a slightly bigger area:

This means that all previously explored areas with slanting edges can now be re-explored to get more map data.