404 pages are blank in production

[stripeyjumper]

Description

The page flashes then goes blank if you navigate to any 404 page in production:
https://next.gatsbyjs.org/thispagedoesntexist

• It happens on my own page hosted on Netlify as well as gatsbyjs.org.
• No errors are thrown on my page, but gatsbyjs.org has some 404 errors for font assets.
• The https://next.gatsbyjs.org/404 page still works ok, so I haven't managed to reproduce it in dev.

It looks like it might be related to #7289. Could it be something to do with the new router?

Steps to reproduce

Navigate to a non existent page:
https://next.gatsbyjs.org/thispagedoesntexist

Expected result

The 404 page should be rendered.

Actual result

The page renders and then disappears.

Environment

File contents (if changed)

gatsby-config.js: N/A
package.json: N/A
gatsby-node.js: N/A
gatsby-browser.js: N/A
gatsby-ssr.js: N/A

[KyleAMathews]

Thanks for the bug report! This will be fixed in #7355