Try mathjax http://www.mathjax.org/ . I personally find it very good.

We migrated from Visual Studio and Markdown

Do you know what was used in your previous workflow to render Math in HTML ? This would help to know how to configure Quarto document. Let me explain why below

I can reproduce your issue with this simple document

---
title: "Math"
format: html
---

$$
\mbox{FMV}_{GAP}^{implied}  = 100* (\frac{\mbox{Implied_Interest_Rate}}{\mbox{FMV_Interest_Rate}} - 1)
$$

By default this is rendered using MathJax 3. But if I change the math method to be Mathml for example, then it works

---
title: "Math"
format: html
html-math-method: mathml
---

$$
\mbox{FMV}_{GAP}^{implied}  = 100* (\frac{\mbox{Implied_Interest_Rate}}{\mbox{FMV_Interest_Rate}} - 1)
$$

It seems Mathjax does not know how to render \mbox{ inside \frac{ as this works

$$
\mbox{FMV}_{GAP}^{implied}
$$

but this does not

$$
\frac{\mbox{Implied_Interest_Rate}}{\mbox{FMV_Interest_Rate}}
$$

It seems like a Mathjax bug, or regression or else as I can make your equation works if you use version 2.7.2

---
title: "Math"
format: html
html-math-method: 
  method: mathjax
  url: https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.2/MathJax.js?config=TeX-AMS-MML_HTMLorMML
---

$$\mbox{FMV}_{GAP}^{implied}  = 100* (\frac{\mbox{Implied_Interest_Rate}}{\mbox{FMV_Interest_Rate}} - 1)$$

About Math in Quarto https://quarto.org/docs/output-formats/html-basics.html#latex-equations

Hope this helps. We can't do much on this unfortunately.