Hi @little_kostas
I’m not sure wether it’s an issue with ACF or not.
But looking at your code I see that you’re not using acfs update_field() function but rather update_post_meta().
In order for field meta to appear properly in newly created posts where the meta hasn’t already existed you have to use update_field() with the field key NOT the field name. Do that and I think you should be fine!
