Insert field codes into equations

I'm a teacher trying to merge data from a spreadsheet (this data is actually
numerical answers to test questions) into a word document equation.

When I insert the field codes into the equation, they initially work when I
preview the results, but then what got merged can never be updated - actually
the field codes are gone after one preview but the original data imported is
permanent text.

This problem only occurs with field codes in formulas, all other data is
imported just fine.

I know that this may be a use of the mail merge feature that was not
originally envisioned, but is there some setting/option that can fix this
problem?