Table of Content [[_TOC_]] in issue comments not rendered

Summary

The [[_TOC_]] syntax does not work in issue comments. There is no Table of Content generated, and instead it just shows [[_TOC_]].

Steps to reproduce

I will copy this entire issue description to a comment below to demonstrate the rendering difference.

They are the same plain markdown content, the TOC in this issue description renders, while the one in the first comment down below doesn't.

What is the current bug behavior?

The [[_TOC_]] syntax is instead rendered as [[_TOC_]].

What is the expected correct behavior?

Should render a Table of Content of that issue comment thread.

Output of checks

This bug happens on GitLab.com as well as my self-hosted instance (13.2.3-ee (640e2695))