@mwesse wrote:
...ultimately knowing how to solve this simple issue is consequential to understanding the loft process in detail
Your understanding of the issue is the problem, as this isn't simple 😉
To our eyes and brains this looks like a simple problem, but under the hood, for the CAD system the loft is not simple as it is very shallow and also very skewed to one side of the geometry.
You might be able to improve the loft surface with a couple of rails, however, a loft from a circular to a rectangular profile will create 4 surfaces, which might look tangent to the naked eye but aren't truly tangent .
When you then try to fillet all edges at the same time, but do not select the edges between these 4 loft surfaces then Fusion 360 creates the fillet you shared in the other thread.
If you do select that edge as well, then you'll end up with a 4-edge situation that sometimes can be solved, but not in this case.