Hi @refreshingdesign
Most likely, the issue is that the function does not work at the time when you are using it in the functions.php file.
Can you check by debuging the value of get_waiting_lists() in the same file as your field group code? Does it work there, or only in single.php?
If my suspicions are correct, you will need to use a filter to set the choices. If you take a look on the docs page, you will see an article which covers this exact query.
Thanks
E
