I have tried to subscribe yesterday using the 50% offer I got in the app and apple took the full amount. I refunded and tried again and I saw that whatever I choose, I never get the discounted amount, it always asks for the full price.
Update: just talked with apple support and they said to basically contact you directly as they are only the middle man. Also to give you the number of the ticket I opened with them: 102523585906
Thank you Sofia. Will post there now. Amounts are not that relevant as I have tried on a different iphone with a different account and the same happened. Different amounts. Apple pay just takes full amount, not the discounted one.