The reason that I say this is that one thing I know about the events calendar is that they use multiple post types and you may not be aware that there are multiple post types. These post types have a complicated parent/child relationship that is not apparent in the admin.
If that plugin is triggering the WP save_post action, but on some other post type then ACF will save the data to the first post added that triggers the action. Then it will not save that values again. So adding something to trigger that action will do nothing at all if it has already been triggered.
What I would do is
// high priority to happen after everything else
add_action('save_post', 'my_save_post_check_function', 999, 3);
my_save_post_check_function($post_id, $post, $update) {
echo 'DID save_post<br />'
if (did_action('acf/save_post)) {
echo 'DID acf/save_post on <br />';
echo '<pre>'; print_r($post);
}
die;
}
What that will do is tell you 3 things,
1) that save_post was triggered
2) that ACF has saved the values
3) what post ACF saved the values against
then execution will stop.
