Ignoring that there are far better and more standard approaches to providing authentication and integrity guarantees, what (if any) weaknesses would a system as outlined below that relies on AES (AES/CBC/PKCS7Padding) encryption to produce a MAC suffer from?
plaintext = 12345678|2018-09-18T00:43:27+00:00 (uniqueID + timestamp)
MAC = aes256(sha256(plaintext))
Could you reliably decrypt the MAC, and then compare the output to the hash of the plaintext to prove integrity/authentication. Assuming that the MAC was generated by a trusted source with a shared secret key.
Also assume that an attacker would have access to lots of plaintext and MAC pairs.