I think that I have found a bug in Halion Sonic 1.5. The damper pedal (on a piano patch i.e.) does not work together with the transpose functionality. The issue can be reproduced the following way (you need to have a damper pedal):
1.) Load a piano patch, i.e. Natural Grand
2.) Press down the damper pedal (this has to be done before playing a note to show the issue), play a note twice and hold it the second time, and release the pedal: The note is still played (because you still press down the key).
3.) Now go to the MIDI settings tab and change the transpose value i.e. to a value of -2.
4.) Press down the damper pedal (before playing the note), play a note twice and hold it the second time, and release the pedal: The note stops to play (even though you still press down the key).
This does not happen every time but quite often.
It would be nice if someone here could confirm this problem.