What characters can break encoding in json?

Question

Yii

There is api for yii2. In the afterSave() method

$record = [
            'reservationId' => $this->id,
            'reservationStatus' => $this->getModeTitle(),
            'message' => $message,
            'emailedGuest' => $emailSent,
            'userId' => $userId,
            'user' => $fullName,
            'type' => $type ? $type : 0,
        ];
        if ($paymentId) {
            $record['paymentId'] = $paymentId;
        }
        if (isset($subject)) {
            $record['subject'] = $subject;
            $record['body'] = $body;
        }
        Yii::info($record, System::LOG_BOOKING);

The problem is that sometimes laman json is stored in the field.
Saves like this

{"reservationId":1981,"reservationStatus":"Balance Receipted","message":"New SMS received from guest","emailedGuest":true,"userId":1,"user":"System Service","type":0,"subject":"SMS","body":"That's great. Thx u. So I could wrk while on holiday

.
Where to look? What could be the problem? Thanks in advance
PS field type "text"