public static Vector2 DistanceBetweenPoints(Vector2 point1, Vector2 point2)
{
    // ...
    if (float.IsNaN(sizeX)) sizeX = 0; // <--- ADD THIS
    if (float.IsNaN(sizeY)) sizeY = 0;
    return new Vector2(sizeX, sizeY);
}

The same logic for the other methods.
Just copy-paste the check line for Y and replace Y to X.

This bug does not require the immediate release of a fix, because you can easily check for NaN on your side.

Thank you for the bug report.
The problem has been fixed.
In the next version this will work correctly.

to calculate the distance between some Vector2 longitude/latitude points.

It work fine for most of my values, but unexpectedly it fail on a couple of points, because it return a Nan value.
I've tried using doubles instead of Vector2 and it return a totally weird value.

I really don't know why, please can you check the picture in attachment and help me to understand what's going on?
Many thanks!