Composer fields empty when revisiting it

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I'm having an issue with Composer.
Whenever I fill in fields in Composer, I am able to save the data and publish a page.

However, if the Composer page is returned to, it will be empty.

For example, if I open Composer on an already published page, the Composer form will be completely empty.

Another example, if I'm creating a page via Composer and click through to something else (such as "SEO"), then when I click "Composer" again all of the data entered so far is lost.

I'm on version 5.7.5.13.