This is because the performance hack to remove all coincident constraints by using the same point. In Arc only the middle point gets tested, but thats the least likely to be coincident to a line.Next steps I think should be a more intuitive creation of the arcs, similar to the function of the draft module and it also seems the coincident constraint of e.g. a line with the end points of the arc doesn't work.
So we have to test the end and start point here....
For the creation:
There are two cases.
Case 1: free creation of an arc.
There you have to define at least three points on the arc or start,end, middle. But in CAD this is rather unusual, cause a arc is
most likely part of a hull in some sense. So creating a free arc means you have to add some constraints by hand..
Case 2: creation tool (like Catia)
Most likely use is the creation of a set of lines and arcs in a row, with some predefined constraints. E.g.
starting with a line. At the end of the line you can decide whether continue with another line or with a tangent arc.
If the arc is tangent you can define the complete arc by the next point alone. Then continue with another (coincident) line or arc.
That makes a fast workflow to define a complete hull of a Part...
If the arc is implemented and logari is not interested to implement such a work flow let me know. Then I will do it!