To demonstrate, I'll use the following markdown file:

---
title: "Some frontmatter"
# A comment
---

# The title

A paragraph.

## A Subheading

Another paragraph.

When opening this file in Helix and pulling up the document symbols, the # A comment line of the frontmatter is treated as a markdown heading, even though it should simply be ignored:

It also shows up in auto-completions: