random notes playing during playback

During playback, my strings sometimes hold notes for extreme lengths, even though they are not notated.
For example, in a quaver C may be held for 20 bars of the piece during playback. I'm not sure if it is due to the staccato marking, or perhaps the fact that I have all Musesounds excluding the strings. It may be affecting some other people, but I'm not sure.

Even when I export the audio with hopes that it will fix itself, it doesn't.