Description

When a child theme is active and includes templates, there is a duplication issue in the Site Editor. Two instances of each template that exists in the child theme are present, with the parent template being loaded in the Site Editor (while the front end is properly using the child theme template).

Step-by-step reproduction instructions

1. Create a child theme with templates and template parts.
2. Activate child theme and access the Site Editor > Templates (or Template Parts) screen.

For testing purposes, I am attaching a zip file with TT3 and a sample TT3 child theme.
tt3-themes.zip

Screenshots, screen recording, code snippet

Environment info

Problem DOES NOT exist with WP 6.2.2 and Gutenberg 16.3 active.
Problem DOES exist with WP 6.3 RC2 (with or without Gutenberg 16.3 active).

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