Iteration Number in JSR223 Groovy string

JMeter has a built-in variable ${__jm__Thread Group__idx} to retrieve the
current iteration, and I want to replace the `Thread Group`
using ${__threadGroupName}. So this comes natural to me:

"Iteration Number is ${__V(__jm__${__threadGroupName}__idx)}"

But when I try it, I always get "0".

What's wrong with it and what's the fix pls? thx