shallow arc problem

 From:  takesh h (TAKESH)
Hi, I have been moiing for about a month now, without any problem, until today.
The problem I encountered today has something to do with arcs.
I was trying to make a four sided shape which comprises of three straight lines and one arc.
However, as you can see in the attached image, no matter how I make this shape (JOIN, BOOLEAN/DIFF, BOOLEAN/MERGE, or BOOLEAN/ISECT) the resulted arc-side ends up being a straight line once a plane is made out of.
If I try the same process with arcs with smaller radius, I get expected results. This happens only when I use very shallow arcs, so it seems.

Anybody has seen this problem before? and possibly some workarounds?