hook_search_preprocess is not preprocessing

1. Fresh D8.3
2. Create 3 articles
3. 1st node with title = Muller
4. 2nd node with title = Müller
5. 3rd node with title = Mueller
6. Run cron to index content.

7. function MYMODULE_search_preprocess($text, $langcode = NULL) {
   
     if ($text == 'muller') {
       $text .= ' mueller';
     }
   
     return $text;
   }
   

8. Search for 'Muller'
9. Search returns nothing

Expected behaviour: Search returns all 3 nodes that we created earlier.