C# AES-256 Encryption
What abc said and also you don't seem to have any IV (Initialization Vector) in you PL/SQL code at all.
The fact that the first part are the same has to do with the different modes (ECB and CBC). ECB encrypts each block separately while CBC uses the previous block when encrypting the next one.
What happens here is that since you use CBC and do not set an IV the IV is all zeroes.
That means that the first block of ECB encryption and CBC encryption will be the same.
(Since A XOR 0 = A).
You need to make sure you use the same encryption mode in both systems and if you decide on CBC make sure you use the same IV.