It might be helpful to know which for version of Moodle you are using, and what you are using to display mathematics in the STACK settings (TeX notation or MathJax)? If it is MathJax, how do you have it included? Do you have MathJax and TeX filter enabled or both? You might look at the healthcheck for STACK to make sure everything is displayed correctly.
I might check also what delimiters are being used. It is possible you changed that option somehow and that may make a difference in whether some questions are displayed.