This sort of looks like what the beams would be, if you wrote out the roll as an 8th note plus all 16ths. Not quite though, I think the 16ths would all be beamed together in each beat. So really, this looks like what the beams would be if it were one 16th and all 32nd notes, except missing one beam. And that would be a decent way of getting MIDI/notation software to play an accented roll if it didn't have the capability of interpreting the tremolo mark.
So my best guess is that this software implements rolls "behind the scenes" as described, but hides that layer, and some glitch has caused part of that layer to become visible.
Either that, or this is some weird visualization meant to show you what the playback is going to do, which wouldn't print.