Apple is not a bank. It does not approve or decline cards for addition to Apple Wallet. Apple is not a credit card processor. Apple cannot approve or decline a transaction. Apple does not change any numbers or details associated with your account.
Apple provides a secure encrypted storage solution for credit and debit cards. Card information is not stored on your device. Your device encrypts the information and sends it to the Payment Network Operator (Visa, MasterCard etc.) and to your bank. Your bank returns an encrypted dynamic token to Apple for storage in the Secure Element.
The dynamic token provides a unique, one use code for each merchant transaction. Only your bank can decrypt the token and approve or decline a transaction.
Probably more than you ever wanted to know about Apple Pay and security.
Any declines are being sent by your bank or the PNO (if the suspect fraud). If it’s not a real decline, it could be the merchant having issues and just called it a decline. You’ll have to ask your bank’s support team.