Some pages are missing from the Parent Page select in the Editor

Two of my top-level Pages, and all of their children, are missing from the dropdown menu in the Block Editor for selecting a Parent Page.

There are no special characters in the titles or permalinks. All of my other Pages do show up in the dropdown menu. And in fact, these two missing top-level Pages are already parents to some of my other Pages. They used to appear in the dropdown, and the database still has their IDs saved. (I do not know when they stopped appearing in the dropdown.)

So for example, if I look in the database at a child Page, the post_parent is set to the correct parent ID. And when I view the child Page or its parent Page on the front end, in the hierarchical left navigation both the parent and all of its children are shown. But when I edit the child Page, it says Parent Page is (no parent).

I've already:

• Checked REST API is enabled (querying the REST API pulls them up accurately)
• Checked postmeta (they have the same postmeta as the working Pages)
• Checked taxonomies (no taxonomies assigned to these Pages, or to working Pages)
• Checked Yoast settings (they are not noindexed or nofollowed)
• Deactivated all plugins
• Switched to Twenty Twenty theme
• Hard-refreshed the Editor each time
• Deleted one of the Pages, re-created it with the same slug, and saw that the new Page appeared as a Parent Page in the dropdown