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".
Jens fixed this in the dev version 8-11 hours ago:
Snap! Build Your Own Blocks
Why do you need brackets?
Because that's what Jens chose.