I think it’s always been broken, at least on bitmap layers (or maybe i just have no idea how to use the tool). Maybe you’re talking about polylines on a vector layer, though, since you mentioned curves. After creating the initial lines you can use the finger/smudge tool to edit points on the curve.
I guess that might still not be what you’re talking about, and so i’ll return to my original comment that i think polyline on bitmap layer has always looked sloppy, probably even before the pencil2d fork. Also the double-click to end the line is the biggest pain, especially when using a tablet (i can’t get my stylus to double-click about 90% of the time).
So anyway, yeah, i think you’re right that this is probably a bug, or at least a feature that needs tweaking at some point.
Bump for never being fixed (at least not in the latest nightly build)
I recently noticed a problem with the tool myself. The first click gets a weird jag and the whole curve afterward is ugly.
I don’t remember having this problem with the original Pencil, and the tool was useful for making smooth curves even if I was using a mouse. Now it’s totally broken, I can’t even adjust like thickness.
Here’s a screenshot, I clicked those black points (I am aware that it was designed to not smooth lines if the corner is too sharp).