When I picked up my iPhone 18 Pro Max in the local Apple Store on Sept 18, I had a scheduled trade-in of my iPhone 17 Pro Max, but needed to transfer content and ESIM between the phones at home. The store manager told me that if I came back into the store within 14 days, full credit of the trade-in would be applied to the credit card. If outside of 14 days, it would not be credited to the card.
I turned in the iPhone 17 Pro Max within 2 days and the full trade-in allowance of that phone was applied to my credit card within three days.
This was an instance where I did the iPhone 18 Pro Max as a pre-order through the online Apple Store (U.S.) with trade-in, and chose to pick up the new phone in the local Apple Store.
Why the process above was not applied in your case — provided you did not omit important details that may have left that store with no choice — remains a mystery as there are no Apple employees participating here to clarify.