For the sake of readability, I've added the example description.ext I use as an attachment rather than pasting it here.
The following excerpts from the BIKI is what makes this seem like incorrect behaviour:
* "Returns value of given scenario attribute from the 1st tier."
* "Return Value: Number, String or Array, depending on the attribute value type. Nil when the attribute is undefined."
Currently, in order to get the value you still have to fall back on the old getNumber/getText/getArray commands combined with getMissionConfig.