Seems like a bizarre one to me, as there is no obvious trigger for either problem.
Basically, occasionally upon reloading a save, one of the tracks becomes abnormally vertically enlarged to what appears to be the size of the full note-range (see the attached image). Once this has happened, the track's notes still sound when the song is played in the song editor, but none of the track can be played from the piano roll, even though the notes still do show up there.
Up to this point, all the issues are merely annoyances. Things get worse when all the track's notes decide to disappear. The first time all this happened, I didn't notice until the notes were already gone; the second time, I watched the notes vanish while the song was playing.
So far, I've been unable to reproduce this at will, as the events truly seem to happen at random, but if I am able to soon I will post the steps in a comment.
Just to be thorough here, I'm still running Ubuntu 11.10, now two versions old. These problems have only started occurring since I updated LMMS (0.4.13-stable.git.2228-4~oneiric1 >> 0.4.13-stable.git.2240-1~11.10) (also: common, vst). Ideas?