ACF stores dates in the database as “YYYYMMDD”, since you are using get_post_meta(), you are bypassing ACF and it will not format you’re dates for you. You’ll need to do this yourself.
To make the columns actually order by this column you need to add a pre_get_posts filter to these posts in the admin, here’s a tutorial I found, and you can probably find others https://wpdreamer.com/2014/04/how-to-make-your-wordpress-admin-columns-sortable/
