Encryption and Decryption in PHP occasionally failing

Question

We're trying to implement an authenticated symmetric encryption in PHP, which works most of the times, but occasionally fails, and we do not really understand why. The code is:

// Encryption of $secret_code, e.g. integer = 152
$secret_code = 152;

$nonce = random_bytes(SODIUM_CRYPTO_SECRETBOX_NONCEBYTES);

// This is just an example, keys are generated in a more secure way, returning a base64
// (Compatible with sodium_crypto_secretbox according to https://www.php.net/manual/de/function.sodium-crypto-secretbox.php)
$key = sodium_base642bin(
'Jdjkfhuehrfkjshdfkjhskjfoshdfishf',
SODIUM_BASE64_VARIANT_URLSAFE_NO_PADDING
);

$cipher = sodium_crypto_secretbox(
random_bytes(8) . $secret_code
$nonce,
$key
);

sodium_memzero($key);

$ciphertext_base64 = sodium_bin2base64(
$nonce . $cipher,
SODIUM_BASE64_VARIANT_URLSAFE_NO_PADDING
);

sodium_memzero($nonce);
sodium_memzero($cipher);

// Decryption then sometimes yields inconsistent values for $secret_code, e.g. 154

$ciphertext = sodium_base642bin(
$ciphertext_base64,
SODIUM_BASE64_VARIANT_URLSAFE_NO_PADDING
);

$nonce = mb_substr(
$ciphertext,
0,
SODIUM_CRYPTO_SECRETBOX_NONCEBYTES,
'8bit'
);

$cipher = mb_substr(
$ciphertext,
SODIUM_CRYPTO_SECRETBOX_NONCEBYTES,
null,
'8bit'
);

$secret_code_string = sodium_crypto_secretbox_open(
$cipher,
$nonce,
$key
);

sodium_memzero($key);
sodium_memzero($nonce);
sodium_memzero($cipher);

$secret_code = (int) mb_substr(
$secret_code_string,
8,
null,
'8bit'
); 

We suspect that the issue lies somewhere in the splitting of the string on an 8bit base and base64 string encoding; we're assuming some data is getting truncated occasionally, resulting in the decryption not to fail, but instead yielding the wrong $secret_code. Any idea?