iPhone 13 Pro Max serial number begins with letter F not M

Serial number or model number? F in the serial number start means retail refurbished by Apple or its contract manufacturers. F at the beginning of the serial number represents the factory location although it's the first two characters.

No, it is not a problem.

If the Model number starts with:

M = New

N = Replacement

F = Refurbished

P = Personalized

The serial number is not involved.

Did you read my reply? The “F” in the serial number is irrelevant.

What is the model number at: Settings - General - About - Model = ??

It means you bought a refurbished phone rather than a new phone if as explained by stedman1 the model number begins with F.