adding templateParts to child theme disables random titles of other template parts

[9-tailed]

Description

The 2022 theme has 4 template parts. I wanted to add 1 with a child theme, a second footer. This worked as intended, but it had a little side-effect. After I added a new template part, one of the parent templates parts lost its title, instead, it showed the slug/name. After adding a second template part another parent template part lost its title but the first one got its back.

Step-by-step reproduction instructions

1. Create a Child theme of 2022 theme
2. add a custom template part to the theme.json

    "templateParts": [  
        {
            "name": "footer-landing",
            "title": "Footer (Landing Page)",
            "area": "footer"
        }

"Header" now losts its title and is shown as "header" in the editor.

3. add another template part to the theme.json

    "templateParts": [  
        {
            "name": "header",
            "title": "Header",
            "area": "header"
        },
        {
            "name": "footer-landing",
            "title": "Footer (Landing Page)",
            "area": "footer"
        }

"Header" is back to "Header", "Header (Dark, large)" is now shown as "header-large-dark"

Screenshots, screen recording, code snippet

Environment info

WP: Version 5.9.1 Gutenberg: Version 12.6.1 Theme: Twenty Twenty Two Child 1.1

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes

[chthonic-ds]

Just noting I've seen similar behaviour when adding templates to a theme, no child required. I didn't have time to troubleshoot at the time but will see if I can replicate when I get a free moment.

[Mamaduka]

I think templateParts of a child and parent theme aren't correctly merged.