This looks to be a bug when you have the transition timeline source instrument (the green one) touching the edge of the destination instrument (the far right see-through handle). To workaround this for now, pull the source instrument back ever-so-slightly until it is no longer overlapping (it will become a single lane size). This will allow the source instrument to fully fade out.
You could also enable “Cut” on the multi instrument which will cause the instrument to untrigger once the playback cursor has exited the transition timeline.
Thank you for bringing this to our attention.