The method docs say it returns an array where key is the module name and the value is the human-readable string, but in fact it returns lang_string object (because lazyload is set to true) https://github.com/moodle/moodle/blob/master/course/lib.php#L566-L567

It works perfectly across Moodle as lang_string has a magic method __toString() https://github.com/moodle/moodle/blob/master/lib/moodlelib.php#L10658-L10665

However, it doesn't work when lang_string is encoded into a json string:

$name = $cminfo->get_module_type_name();

echo('Mod name: ' . $name);

echo('Mod name json: ' . json_encode($name));

has the following output:

Mod name: Assignment

Mod name json: {}