The mathematical expression part editor doesn't pick up variables used in expressions substituted into the expected answer

I have a question variable `answer = expression("x")`. The expected answer for the part is `{answer}`. The variable `x` doesn't show up in the list of variable value generators in the accuracy tab.

Reproduce the question with answer = expression("x") and expected answer {answer}, then inspect the mathematical expression part editor's accuracy tab and its variable value generator list. The issue is done when x appears as a variable value generator for this substituted expression.

Written by the indexing model from the issue text.