Some complicated math expressions don't render in markdown_dollar_math mode:

Summary

Some complicated math expressions render using the old math syntax but not the new markdown_dollar_math syntax.

Steps to reproduce

The following works:

\dot{X} = \underbrace{\frac{A}{\rho B}}_{\text{first term}} + \underbrace{\frac{C (D - E)}{F \gamma}}_{\text{second term}} + \underbrace{\frac{h_{i} (J_{k} - L)}{M_{\lambda}} }_{\text{third term}}

```math
\dot{X} = \underbrace{\frac{A}{\rho B}}_{\text{first term}} + \underbrace{\frac{C (D - E)}{F \gamma}}_{\text{second term}} + \underbrace{\frac{h_{i} (J_{k} - L)}{M_{\lambda}} }_{\text{third term}}
```

while the same thing with double dollar sign syntax does not.

$$ \dot{X} = \underbrace{\frac{A}{\rho B}}{\text{first term}} + \underbrace{\frac{C (D - E)}{F \gamma}}{\text{second term}} + \underbrace{\frac{h_{i} (J_{k} - L)}{M_{\lambda}} }_{\text{third term}} $$

```math
\dot{X} = \underbrace{\frac{A}{\rho B}}_{\text{first term}} + \underbrace{\frac{C (D - E)}{F \gamma}}_{\text{second term}} + \underbrace{\frac{h_{i} (J_{k} - L)}{M_{\lambda}} }_{\text{third term}}

Double dollar sign syntax does work with only a single underbrace term:

$$
\dot{X} = \underbrace{\frac{A}{\rho B}}_{\text{first term}}
$$

\dot{X} = \underbrace{\frac{A}{\rho B}}_{\text{first term}}

Underbraces seem scary but the same issue arises with bold terms where this works:

```math
\dot{X} = \bold{\frac{A}{\rho B}}_{\text{first term}} + \bold{\frac{C (D - E)}{F \gamma}}_{\text{second term}} + \bold{\frac{h_{i} (J_{k} - L)}{M_{\lambda}} }_{\text{third term}}

```math
\dot{X} = \bold{\frac{A}{\rho B}}_{\text{first term}} + \bold{\frac{C (D - E)}{F \gamma}}_{\text{second term}} + \bold{\frac{h_{i} (J_{k} - L)}{M_{\lambda}} }_{\text{third term}}
```

and still, the same thing with double dollar sign syntax does not.

$$ \dot{X} = \bold{\frac{A}{\rho B}}{\text{first term}} + \bold{\frac{C (D - E)}{F \gamma}}{\text{second term}} + \bold{\frac{h_{i} (J_{k} - L)}{M_{\lambda}} }_{\text{third term}} $$

```math
\dot{X} = \bold{\frac{A}{\rho B}}_{\text{first term}} + \bold{\frac{C (D - E)}{F \gamma}}_{\text{second term}} + \bold{\frac{h_{i} (J_{k} - L)}{M_{\lambda}} }_{\text{third term}}

There also appears to be an issue with repeated less-than signs: $0 < a < 1$ (`$0 < a < 1$`) doesn't work, but it does work with backticks, as with: $`0 < a < 1`$.

Thanks for the help!

### Example Project

<!-- If possible, please create an example project here on GitLab.com that exhibits the problematic 
behavior, and link to it here in the bug report. If you are using an older version of GitLab, this 
will also determine whether the bug is fixed in a more recent version. -->

### What is the current *bug* behavior?

Only renders with the old syntax.

### What is the expected *correct* behavior?

All the above examples should work in both cases.