Ruben

I found out the issue. It was because the product is a variable product but has no variations.

Tried it and it still doesn’t work:

$new_stock = wc_update_product_stock($post_id, $qty, 'set', false);
wc_delete_product_transients($post_id);

I see this:
Out of stock (8)
Out of stock (3)
etc..

Thank you very much, lorro!
I will try that and get back to you if it doesn’t work.