Checklists are broken

Create a post with checklist items:

It saves and renders without the checkboxes:

Then I reload the page:


I complete a checklist item:

Then I reload the page:

If I complete another checklist item:

But then do not reload the page, I am left in a broken state, such as if I try to add another item (or delete one):

I get this error:

Reproduce the checklist workflow in a WordPress post: create items, save, reload, complete items, and edit again without reloading. Trace the checklist editing and rendering path from the post editor, then verify that checkboxes persist through saves and reloads and that adding or deleting items no longer leaves the editor in an error state.

Written by the indexing model from the issue text.