weird behaviour in jit.gl.sketch with message framecircle ?

There's something weird with the way framecircle message to
jit.gl.sketch works :
If one make a framecircle with, say:
- theta start = 180
- theta end = 360
The frame is slightly shifted, angle wise, up to 15 degrees.
This phase shift increases with the angle value.

I dont know if I make myself clear, but it is easy to observe this issue
with the patch below.
I found no hint in the forum, nor in OpenGL documentation... Anyone knows ?