After investigating a bit more, adding noLoop() at the end of draw() will draw the curve correctly. It looks like the renderer doesn't break the curve drawn in the current frame from the curve drawn in the previous.

Don't repost, even less in the wrong section of the forum. This is not a Library and Tools Development post (you don't develop a library or a tool, right?), but indeed a Core issue (OpenGL is a core library).
I deleted the duplicate message, repost here if you want to bump up and/or add additional information.

I don't see your problem in Processing 1.5.1. Don't forget that 2.0a as a 'a' to mean 'alpha', ie. to mean it is full of bugs... If you think it is a bug, just go to the Code section of the site and report the bug (after checking it isn't reported already).

I don't have 2.0 yet, but you can try and add a background() call at the start of draw(). If it often a source of trouble to forget it (unless the sketch specifically needs cumulative drawing).

For some reason my brain associated Library and Development with OpenGL (a library in the code repo) and 2.0a (in development).

Background() doesn't/shouldn't fix this, so I'll report the bug.

I was hoping for someone who might have found a workaround, which is why I posted here in the first place.