contentSize is only working for a second and then reverting to full width in the editor. WordPress theme.json

Question

I'm trying to set the content size in a new block theme in WordPress using a theme.json file. When I load a Post/Page editor, it appears like it's going to work for a second, but then everything reverts back to full width except for the title. The title keeps a max-width of the settings.layout.contentSize. I'm using WordPress 6.4.2.

I can also see that the class editor-styles-wrapper is declaring CSS variables with the correct pixels, but they aren't being used anywhere.

{
  "version": 2,
  "settings": {
    "layout": {
      "contentSize": "840px"
    }
  }
}

Thank you in advance for any ideas on how to fix this!