I hope this image makes it clear:
The "JOIN" block genuinely reports a ringed "left OF Stage"... This isn't a bug, it's a strange feature. Maybe an experiment to do with macros?
No, the bug is that "left" there is trying to get the stage's "left" variable, that doesn't exist. I'm trying to get the dropdown input "left".
Why do you need brackets?
Because that's what Jens chose.