decrypt() returning incorrect result

I'm converting an app from CI 3.1.13 to CI 4.3.1

In CI3 I had:

PHP Code:

$CI =& get_instance();
$this->api_key = $CI->encryption->decrypt($CI->Appconfig->get('mailchimp_api_key')); 


In CI4 I have:

PHP Code:

$encrypter = Services::encrypter();
$this->api_key = $encrypter->decrypt($config['mailchimp_api_key']) 


In Config/Encryption.php I have:

PHP Code:

public string $key = REDACTED; //32 Character encryption key
public string $driver = 'OpenSSL';
public int $blockSize = 16;
public string $digest = 'SHA512';
public bool $rawData = false;
public string $encryptKeyInfo = 'encryption';
public string $authKeyInfo = 'authentication'; 


Between the two version the key has not changed and the digest value hasn't changed. When I step through the code I see that the decrypted result is garbage characters. I see that it's identifying the encryption algorithm as aes-256-ctr. I don't know if that's the correct algorithm or not from what CI3 was using, but something is off. From reading another forum post, I thought this was CI3 compatible as of CI 4.3.0.