Is AES-256 consistent?
AES-256 is a function from a 256-bit key and a 128-bit plaintext block to a 128-bit ciphertext block.  Same key and plaintext, same ciphertext.

But it is unlikely that your application uses AES-256 directly—most applications don't, because most applications are concerned with encrypting messages, not with permuting the set of strings of 128 bits.  From the information you've given, there's no way for us to know how you use AES-256, or even if you do use it directly, whether there's an application bug that just causes some wrong IP address to be stored or retrieved or causes some IP addresses to be dropped on the floor.