That is correct. The MATH Expression parameter is actually a full text editor field, not a single element field like all the other instructions (except for the STRPRINT/EMAIL fully text editor script parameter). Hence, it has a huge number of permutations and combinations of math functions, nicknames, element names, potential nicknames, parameter lists, operators, etc., so it cannot behave like a simple element field.
We can definitely add an option for resolving potential new nicknames.