This has been detected by MDL-76444 (inspecting apache logs before PHP 8.3 / Moodle 4.4 release). It was called "Problem W" there.

Apparently, it happens when, from the course view page, some stealth stuff or page is loaded. Here it is the stack trace:

112721:[Fri Mar 08 19:22:24.280966 2024] [php:notice] [pid 3886] [client 172.25.0.13:35076] Debugging: Invalid get_string() identifier: 'cm_stealth_feedback_batch' or component 'core_courseformat'. Perhaps you are missing $string['cm_stealth_feedback_batch'] = ''; in lang/en/core_courseformat.php? in

• line 355 of /lib/classes/string_manager_standard.php: call to debugging()
• line 108 of /lib/external/externallib.php: call to core_string_manager_standard->get_string()
• line ? of unknownfile: call to core_external::get_string()
• line 253 of /lib/external/classes/external_api.php: call to call_user_func_array()
• line 83 of /lib/ajax/service.php: call to core_external
  external_api::call_external_function()
• line 33 of /lib/ajax/service-nologin.php: call to require_once()
  , referer: http://webserver63976c3e94796505/behatrun2/course/view.php?id=102000

(it happens once per behat execution, estimated)

It has been detected in main (4.4dev) although it may affect to previous (stable) branches too.