theme.json customTemplates with child themes work wrong

[thomask]

If you use child themes, then customTemplates in theme.json does not work and behaves badly

1. It is not possible to delete or overwrite parent custom template - neither false nor empty array helps
2. It is not possible to create a new custom template - it simply does not appear
3. and what is strange - if you try to create your own custom template, it will throw away the naming and parent template at that position, so then when you select the template for the page, it doesn't show the title but the name. Example - if you make a child theme from twenty-three, and you type

	"customTemplates": [
		{
			"name": "no-title",
			"postTypes": [
				"page"
			],
			"title": "No title"
		}],

so when the page is selected, it will display 'blank' instead of 'Blank' (blank is the name of the first template in the parent twenty-three theme). If you try

	"customTemplates": [{},
		{
			"name": "no-title",
			"postTypes": [
				"page"
			],
			"title": "No title"
		}],

so when you select a page, it will display "blog-alternative" instead of Blog (Alternative) (blog-alternative is the second template in the parent theme).
Child template can't be added like this even in the fourth position

Unfortunately, this makes the whole thing completely unusable.

[thomask]

This is an example of that mentioned third problem

[thomask]

that wrong named parent template is shown also in the left list of templates on site-editor.php