TOC is missing when content imported via mdx

[msteller-connyun]

🐛 Bug Report

When an mdx file is imported into an other mdx file, the imported content is ignored by the TOC.

To Reproduce

1. Create a mdx file with some headlines (#, ##)
2. Import the mdx file into an other one and call the mdx file as a jsx component

main.mdx:

import Demo from './demo.mdx‘;

<Demo></Demo>

Expected behavior

The headers from demo.mdx should be visible in the TOC of main.mdx

Actual Behavior

TOC ignores demo.mdx, when imported.

Your Environment

"@docusaurus/core": "2.0.0-alpha.71",
"@docusaurus/preset-classic": "2.0.0-alpha.71",

[slorber]

Yes, this is a known issue I'd really like to get solved, but it does not seem easy to solve :'(

Duplicate of #3915