What happens is, if you join the choir somewhere around the midpoint, even if you hit the right notes at the right time, it will still damage you.
You’ll also get zero credit for it.
The OP is correct. Just experienced this for myself and it was not fun.
Edit: Had to relog in order to fix this. Don’t know if it matters, but I had the menu up to choose which stage. Clicked off that to then choose the part I wanted. Each time, the event had already started, and each time it bugged.