I’m using a Reverse proxy example.com/blog to point to blog.example.com

Everything works, except forms in the Admin panel. WordPress code is:

<input type="hidden" name="_wp_http_referer" value="/wp-admin/edit.php?post_type=page">

which sends the form to the wrong URL:

example.com/wp-admin/edit.php?post_type=page

while it needs to go to:

example.com/blog/wp-admin/edit.php?post_type=page

I guess the URL is determined by this piece of code (https://developer.wordpress.org/reference/functions/wp_admin_canonical_url/)

$_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI']

Any idea how to fix this without hacking into the core files?