Purge all cache does not purge customised language strings

"Purge all cache" should purge all cache, but it doesn't. Locally translated strings remain cached.

1. Translate some lang strings locally, with or without "Cache all language strings" activated
2. Delete the translated string in the appropriate lang file or the entire file
3. Purge all cache

*) When encountering this issue I had "Cache all language strings" on the pageadmin/settings.php?section=langsettings deactivated, but I am not sure whether the setting was activated when I began to translate the local strings.

See: https://moodle.org/mod/forum/discuss.php?d=335811

Additional info:

In an old Moodle installation I have several strings that are not present in the English nor in the Danish language pack. Nevertheless these strings occur on the page /admin/tool/customlang/edit.php?lng=da. These strings are no longer in use in Moodle (currently 3.4.1+).

Eg. report_security | check_passwordsaltmain_name.

Report_security.php have had no strings with an identifier containing 'passwordsalt' since Moodle 2.4. Neither the English nor the Danish report_security.php in my Moodle instance contains strings with 'passwordsalt', but they occur on the page admin/tool/customlang/edit.php?lng=da. And I can not get rid of them. I can purge all cache, delete the folders /moodledata/cache and /moodledata/localcache, delete my browser cache, and have Site administration → Language → Language settings → Cache all language strings deactivated all together, but the strings remain in Moodle!