Can't get setFieldValue to work

Question

I've got this

    // Creating users through frontend
    Event::on(Users::class, Users::EVENT_BEFORE_ACTIVATE_USER, function(UserEvent $event) {
        $userId = $event->user->id;

        // Allow assigning user groups through the frontend (circumvent elevated user sessions)
        $groupId = Craft::$app->getRequest()->getBodyParam('userGroup');
        if ($groupId) {
            Craft::$app->getUsers()->assignUserToGroups($userId, [$groupId]);
        }

        // This does not work
        Craft::$app->users->getUserById($userId)->setFieldValue('discountPercentage', 15);
    });

Assigning the user group works fine, so I don't get why assigning a field value doesn't. The discountPercentage field is a simple Number fieldtype

I also tried $event->user->setFieldValue('discountPercentage', 15); without success.

Also I tried surrounding 15 in quotes, like '15', but that didn't work either.

Edit:

Submitting the "create user form" through the frontend gives no errors. discountPercentage is just left blank.