That’s because that field doesn’t accept any valid JS expression, but explicitly just the name of a single JS variable (which AB.One['Alpha'] isn’t).
But AB.One.Alpha works, because that case is explicitly supported in the code as you can see in the Javascript that is executed when you create a JS variable in Tag Manager:
Hi @Lukas thanks a lot for your reply! In the meantime (one week after my post, but 4 hours before you wrote your explanation!) I reported this as a bug: [] javascript notation not accepted by MTM
So if I understand you correctly it’s not a bug. The tag manager has this help field:
Reading it again I understand it, but back then I did not understand it to exclude the [] notation. Maybe it’s obvious to anybody who is more familiar with JS. Or it would profit from explicitly ruling that out, like adding “Other notations like document[‘referrer’] are not accepted.”
Daniel