There is an active, correctly configured promotion of type "Promotion By Parent Plans". When the customer with already purchased subscription based on the parent plan attempts to buy a subscription based on the promoted plan, the promotion is not applied, the price stays default, configured in the subscription period. Why?
The promotion of type "By Parent Plan" is only applied when the customer purchases two subscriptions at once, when the parental subscription is assigned at the time of purchase. There is a feature request with id #PBA-52097 ("Promotions by purchased plan") to make the described scenario work.