Hi Marius, I'm responding here again, for the benefit of others who run into this scenario.
Thanks for putting out this post.
The geometry itself is not getting altered, but the circular arc is so flat that the geometry engine is not able to recompute the radius with any degree of precision that would return a meaningful value. If you use edit vertices, and move one end of the line you can see that the segment is still a circular arc.
There is no discernible difference between the chord length and arc length, and the chord height is 0 (or very close to it).
You can use the offset tool to see this:
This is one of the reasons that it’s important to use/store the COGO attributes separate from the geometry. This is unfortunately a case where the geometry is not mathematically stable enough to get meaningful values, and so it thinks there is a straight line here…
You can manually enter the radius and the arc length into the COGO attributes, but then you do need to be careful not to overwrite those again by using the Update COGO tool. For a future release we need to make the Update COGO tool smarter to avoid this problem where the COGO attributes are overwritten with straight line values for very flat circular arcs.
The geometry engine starts to consider the circular arc to be a "straight line" for a central angle of around about 2 degrees.
In this case the central angle is a lot smaller than that:
(5.67/1640) radians x 180°/pi
0.00345732 radians x 180/pi = 0.198 degrees