Just by turning on twig debug and looking at theme suggestions, you can see for the main menu (and any corresponding menu like footer) that there is a duplicate theme suggestion and at the most specific level which overrides any and all customizations in hook_theme_suggestions_alter().
<!-- THEME DEBUG -->
<!-- THEME HOOK: 'menu__main' -->
<!-- FILE NAME SUGGESTIONS:
x menu--main.html.twig
* menu--main--customization.html.twig
x menu--main.html.twig
* menu.html.twig
-->
Should be:
<!-- THEME DEBUG -->
<!-- THEME HOOK: 'menu__main' -->
<!-- FILE NAME SUGGESTIONS:
x menu--main--customization.html.twig
* menu--main.html.twig
* menu.html.twig
-->
