Issue with custom post types and classic editor - "Changes you made may not be saved"

[tonysandwich]

Since upgrading yesterday to 5.6.1, any time I try to edit a custom post type without the Gutenberg editor enabled in the supports array of the custom post type I get a "Changes you made may not be saved" alert, even if I did not make any changes, or even if the post has been published.

Enabling Gutenburg in supports fixes this issue, however if I enable the Gutenburg editor, and then install the classic editor plugin I get the same alert each time I try and navigate away from the post editor screen.

I have reverted back to 5.6 and everything works as it should.

[vladytimy]

Duplicate of #52440.

Hi there! Thanks for reporting this!

This issue has already been reported, and is being tracked in 52440