Mortgage Approval in Principle blocked due to car payment installment descrepancy

I bought my car 1 month back from a dealer ABC in Ireland with 18000 euros amount. On the first day the dealer asked me to pay 1000 for booking the car to the dealers account. He then said he will deliver the car next week. So went there next week, paid 9000 euros as 1st installment to the dealer. It was an online payment with a card reader to an account with different name than the dealer. I was able to take my car. After another week made the 2nd and final installment to yet another account provided by the dealer (with different name than the dealer). Everything and every document with the car is fine. The dealer gave me a receipt for the 18000 euros payment with all my car details.

I am now trying to get a mortgage approval from a bank who have flagged this three debit transactions from my account and want me to get separate receipts for each of the transactions.

The transactions were like
1000 ABC motors
9000 DEF motors
8000 GHI motors

I went back to the dealer who says that they have given receipt for the full amount of the car

Can anyone tell me why is this flagged by the bank? Why are they concerned with specific accounts that the dealer uses? Also how can I get out of this?