That doesn't surprise me. When you specify the coordinate for the point, there is nothing to indicate whether you prefer the beginning or the end of the line. So it returns the solution closest to the beginning of the line.
There are more quirks in here, too. For example, using the option to find intersections on a polyline may return unexpected results at/near vertices if you use the option to extend the line. And for Civil 3D users, these functions work differently on 3D-Polylines than on Feature Lines or Survey Figures (they use the 3D length on 3D Polylines, but the 2D length on the Civil 3D elements.) There's a similar problem with GetParameterAtDistance.
In my experience, this whole area of Autocad is full of gremlins. But at this point, there's an awful lot of other code all built on this stuff, so I don't really expect to see things change. The only advice I can give is to do lots of testing of your code. I've been able to work with all this stuff and can generally accomplish what I want, but there are a lot of quirks to discover.