Faliure to generate TOC if the page contains <h1> in a code fence

[ChenKB91]

Hello, I was writing an article that explains some PHP code, and found this issue.

Steps to reproduce

Generate a page using the following markdown code:

{% toc %}
## aaa

### bbb

## ccc

```
 <h1>ddd</h1>
```

The generated TOC will only have a single entry "ddd". Is this an expected behavior?