You need to set the priority of the format value filter to >10 so that it runs after ACF formats the value because your changes may be ignored it you try to do it before acf runs and priority 10.
ACF formats a select field by using
implode(', ', $array);
What you need to do is to format the array the way you want it to appear. For example if you want the value to have breaks after each value you could do
$value = implode('<br />', $value);
or if you wanted it to be a bulleted list you could do something like
$value = '<ul><li>'.implode('</li><li>', $value).'</li></ul>
