1) You could try temporarily switching the soft synth to something different, to see if the issue is with the track itself, versus some kind of glitch with that instance of TTS-1.
2) Perhaps you could insert a new audio track, and set its Input to the output of the TTS-1, to see if it is an issue with the audio track.
3) Or, insert a new midi track, then copy/paste all of the Events from the original midi track into the new one.
4) Take a look at the audio track OUTPUT assignment for the current audio track, to make sure it is set to Master, or some valid bus.
I don't think I have enough brain left for additional thoughts, at this horrific time of the morning I happened to be up and looking at things (4:37 AM), and it is way too early for any coffee, but maybe something in the above will correct your issue.
Bob Bone