Ah, I see what you meant. It is not a bug, it is working as intended.
Short Answer: It's not really a bug actually, use the action "Force start" instead of "Start".
Long Answer: Play once mode ignored subsequent start call if it is in mid-tween. It is by design, and have something to do with relative mode (geez, relative mode really brings alot of trouble). If you use Force start, it will be played regardless of the tween state.
I hope it works for you, I tried it on my computer and it works fine here.
PS: Very nice work there with the game :>