Encrypt and decrypt of Field

I think the issue is with the mapping of $Amount.ciphertext to $AmountType (in order to map that, the “Encapsulate output” option of the Encrypt Field snap needs to be unchecked). I built a test pipeline that looks like this:

The JSON Generator has simple JSON like this:
[{"msg":"Hello world!"}]

Similar to your setup I used a Passphrase-based Key account type with a simple passphrase.

Here is the Encrypt Field snap:

Here is the Decrypt Field snap:

After validating the pipeline here is the Encrypt Field output:

[
    {
        "msg": {
            "transformation": "AES/GCM/NoPadding",
            "iv": "yjvD0qXH0MuwVqKhaDnZbg==",
            "type": "STRING",
            "ciphertext": "6aJAEWHFctinn8pZUPPLsG+E1OsMH49gEy08xQ==",
            "key_params": {
                "passphrase": {
                    "key_gen_iterations": 10000,
                    "key_gen_algorithm": "PBKDF2WithHmacSHA1",
                    "key_algorithm": "AES",
                    "key_salt": "NB8NMQyEqeItdu0tyn8XTga+udA=",
                    "key_size": 128
                }
            }
        }
    }
]

If there is no Mapper snap between the Encrypt Field and Decrypt Field it works just fine, however, if you add a Mapper after validation you can see the Decrypt Field Snap is expecting more information than just the cipher text (look at the target schema on the right):