This is happening to me too after updating WP to 5.6.1.
Is there any update on this item? Just encountered it myself.
Edit: Nevermind! Found a solution:
function acf_smart_dates($field) {
if ($field['value']) {
$field['value'] = date('Ymd',strtotime($field['value']));
}
return $field;
}
add_filter('acf/prepare_field/type=date_picker','acf_smart_dates');
From: http://blog.room34.com/archives/5597
Just to add my voice to the chorus, I experienced the same issue on multiple flex content items. Rollback to 5.10 solved it.