Is there a hook / action that is triggered when adding or removing a post thumbnail?

I think you want to use the added_post_meta hook instead of updated_post_meta because you’re not updating the meta here, only adding it. At least in the case of the _thumbnail_id, where we must delete it before adding it again (no update) through the admin UI.

Investigating this further we see that this part of the update_metadata() function:

if ( empty( $meta_ids ) ) {
    return add_metadata($meta_type, $object_id, $meta_key, $passed_value);
}

is causing you the problem, because it calls add_metadata() and returns it, before the update_{$meta_type}_meta and updated_{$meta_type}_meta hooks are ever fired.

You therefore need to hook into the add_metadata() function, instead of the update_metadata() function, through e.g. the add_{$meta_type}_meta (before) or added_{$meta_type}_meta (after) hooks.

If we check out the wp_ajax_set_post_thumbnail() function, that’s ajax-requested from the admin UI when adding/removing the featured image, we see that it uses the functions
set_post_thumbnail() and delete_post_thumbnail().

The latter one is a wrapper for delete_metadata(), that fires up the delete_{$meta_type}_meta (before) and deleted_{$meta_type}_meta (after) hooks.

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