Laravel Eloquent 'updated' event - when are relationships updated based on dirty attributes?

I have a Laravel Eloquent model observer observing the updated event on a given model, looking for changes in certain values.

If the value has changed I want to perform some action.

The abridged code:

// Check the lessonRequest to see what's changed
if($lessonRequest->subject_id !== $lessonRequest->getOriginal('subject_id')){
    $msg =  __('app.lesson_request_subject_changed', ['name' => Auth::user()->name, 'subject' => $lessonRequest->subject->title]);
    $lessonResource = LessonResource::createChatMessage($lessonRequest->id, $msg, null);
    $lessonResource->save();
}

Unfortunately, the subject relationship on the $lessonRequest object still has the previous subject, rather than the record defined by the ID that was updated.

Is this deliberate behaviour, or have I done something wrong?

via Chebli Mohamed