I've been having bad luck testing new functionallity lately - the first thing I try never works
I finally figured out that if I lowercased "MB" to "mb", then it works as expected.
Also, it looks like because ignoreCase is true in the initArgs, everything is stored as lower case in the managed map (losing information). Ideally case would be preserved in the synonym file (esp if one wanted to change ignoreCase either way). I imagine stopwords may have the same issue, but I haven't checked.