Hi Andre, it's kind of both. It's a known rendering bug but it has intentionally not been fixed as of yet since the fix could cause some performance degradation for all curve drawing.

See here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=4101.2

At some point I should be able to do a careful fix but I'm not sure exactly when. Thanks for reporting it.

- Michael