heading styles are not applied as intended

[georg94]

In a docz project I tried to change some styles and encountered some buggy behavior of the theme.
The current theme already applies styles to headings

https://github.com/doczjs/docz/blob/main/core/gatsby-theme-docz/src/theme/styles.js

{
// ...
h1: {
    fontSize: 6,
  },
  h2: {
    fontSize: 5,
  },
  h3: {
    fontSize: 4,
  },
  h4: {
    fontSize: 3,
  },
  h5: {
    fontSize: 2,
  },
  h6: {
    fontSize: 1,
  },
// ...
}

These styles are not applied to the heading tags in the DOM (no class is appended).

I debugged this issue and found this commit a9384b0.

Removing the MDXProvider from the Layout fixes the issue, but I don't know if it breaks something else. In my project everything is still working (and the styles are applied).