Problems when you define a site_url that is different from the home_url

[miscs]

Description

I use WordPress in a headless mode and therefore I define a site_url that is different from the home_url in Settings->General.

home_url: backend.domainA.com
site_url: domainB.com

If have seen #1761 and implemented the mentioned workaround

add_filter('rest_url', function($url) {
    $url = str_replace(home_url(), site_url(), $url);
    return $url;
});

With this filter I can use Gutenberg again (add / edit posts). But when I use a block theme, I see the following call to the site_url, which obviously cannot work:

site_url/?p=10&_wp-find-template=true&_locale=user

Step-by-step reproduction instructions

1. Go to Settings->General
2. Define a site_url which is different from home_url
3. Wonder why this is not supported by Gutenberg and implement the workaround in Gutenberg breaks completely if site URL is not the same as wordpress URL #1761 using the rest_url-filter
4. Open Gutenberg and check console -> there are errors logged because of a request to site_url/?p=10&_wp-find-template=true&_locale=user

Disclaimer: I want to use a block theme in the backend because of the functionality it provides in wp-admin, even when I am using WordPress in Headless mode with a custom frontend.

Screenshots, screen recording, code snippet

No response

Environment info

• WordPress 6.3
• Twenty Twenty-Three
• No other plugins

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

[github-actions]

Hi,
This issue has gone 30 days without any activity. This means it is time for a check-in to make sure it is still relevant. If you are still experiencing this issue with the latest versions, you can help the project by responding to confirm the problem and by providing any updated reproduction steps.
Thanks for helping out.

[miscs]

It is still relevant with latest release.

[github-actions]

Hi,
This issue has gone 30 days without any activity. This means it is time for a check-in to make sure it is still relevant. If you are still experiencing this issue with the latest versions, you can help the project by responding to confirm the problem and by providing any updated reproduction steps.
Thanks for helping out.

[dz0ny]

Here is a partial solution.

#57945 (comment)