ACF Custom validation message not showing up

I have two date fields where I want to make sure the “end date” is always later than the “start date”.

I added a validation function per ACF’s documentation. It does fire, but the custom message does not display next to the field. It only says “validation failed” on the top.

add_action('acf/validate_save_post', 'my_acf_validate_save_post');

function my_acf_validate_save_post()

    $start = $_POST('acf')('field_5fb0e816ea4fc');
    $start = new DateTime($start);

    $end = $_POST('acf')('field_5fb0e83aea4fd');
    $end = new DateTime($end);

    // check custom $_POST data
        if ($start > $end) {
            acf_add_validation_error('event_series_end_date', 'End Date should be later than the Start Date');