Autoclass directive works in rST but not Markdown

[felixvd]

Describe the bug

When I try to use autodoc directives like autoclass in Markdown format, they do not work properly. If they are used inside an {eval-rst} block, they work fine.

Comparing the output looks like this:

I may be making a formatting or other mistake, but {note} and other directives appear to work fine.

Reproduce the bug

1. Create a Markdown file with this content, where the autoclass directive is used once inside an eval-rst block which succeeds, and once as a directive from MyST as described here, which fails.
2. Build it with Sphinx and Myst enabled.
3. It will look like the screenshot further up.

List your environment

Sphinx 1.8.5, myst-parser 0.15.2

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