We recently noticed an issue that WordPress wouldn’t register new PHP patterns in an FSE theme. This mostly occurred in our local environments via DevKinsta and another dev was using Laravel Herd. After a few hours of testing, we discovered that adding the following to wp-config.php solved the issues. It appears that some aggressive caching was causing the problems.
define( 'WP_ENVIRONMENT_TYPE', 'development' );
define( 'WP_DEVELOPMENT_MODE', 'theme' );