Not sure if it will fix it but I noticed that your remove_action call doesn’t specify the priority of the function. From the codex ( https://codex.wordpress.org/Function_Reference/remove_action ):
Important: To remove a hook, the $function_to_remove and $priority arguments must match when the hook was added. This goes for both filters and actions. No warning will be given on removal failure.
In other words try
remove_action( 'acf/save_post', 'my_post_updater', 20 );
