Consider the following solutions:

1. When creating the post, make sure to use update_field() for the event_date field. This function saves the value in a way that ACF and WordPress recognize.

2. Re-save the post meta with acf_update_value:

   $event_date = '2024-01-01'; // Example date

   acf_update_value($event_date, $post_id, 'event_date');

3. If the event_date is stored as YYYYMMDD (the ACF default) or Y-m-d, ensure that the value set programmatically matches this format. Passing an incorrectly formatted date can cause issues with sorting if ACF expects YYYYMMDD.