Single transaction split into multiple and I lost Amex Offer

I used 2 cards to pay for my order. Each card is supposed to be charged $650/- as I had split the payment and I’m supposed to receive cash back on each one of them. Now on each card the transaction was split into 2, one for $501/- & one for $149/-. The same thing happened on both the cards. Now I had lost the cash back from my credit card as it’s supposed to be single $650/-on each card. Why did my Single transaction split into multiple and I lost Amex Offer.