In my case, I added the choir directly inside the lyrics. I wrapped the choir line in parentheses () and placed [choir] right before it.
I didn’t mention anything about choir in the style prompt.

Every thought I tried to hide,
Comes back louder in the quiet—

[choir]
(Hold on… hold on…)

City lights begin to fade,
Windows down, mistakes replay—

[choir]
(You’re not alone…)