Trendyol Denied My Free Product Despite Their "Buy 5 Get 1 Free" Campaign

I made a product request from the KoreaVit brand's "Buy 5, Get 1 Free" campaign on Trendyol. Using a tactic that the seller applied to everyone, they sent me only 4 items instead of 5. When I asked why, they claimed that the "Vitamin C" could not be supplied. However, it is now available to add to the cart. They couldn't send it and issued a refund. I reported to Trendyol customer service how I could repurchase the ₺468 product with a ₺96 refund. In this situation, while an intermediary organization might analyze the issue and think that the seller should cancel the sales contract, Trendyol is just backing them up, saying there is nothing they can do. It seems illogical and unreasonable to dismiss a customer's legitimate claim just to protect the seller. It's really disappointing.