variable size array of uint8 as msg field has strange values

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Hi everybody, i'm using MKR1300 arduino board and rosserial. I have this msg type:

uint32 id
uint8 lenght
uint8[] value
uint64 timestamp

These fields should contain information on CAN type packages. In the field value I am inserting the sequence of bytes in this way (arduino side):

void AddOneToROStail(int packetSize){

  int i = 0;

  if(countR == BUFFER_LENGHT){
    Serial.print("Overrun from CAN..losing packets");
    return;
  }

  buff_toROS[headR].id = CAN.packetId();
  buff_toROS[headR].len = packetSize;
  buff_toROS[headR].timestamp = now();

  //get the raw bytes
  buff_toROS[headR].value = new unsigned char[packetSize];
  for(i=0; i<buff_toROS[headR].len; i++){
    buff_toROS[headR].value[i] = CAN.read();
  }

  headR = headR + 1;
  headR = headR%BUFFER_LENGHT; //make sure head [0 % BUFFER_LENGHT-1]
  countR = countR + 1; //new alement in the queue  
}

So when a package arrives I fill a struct and immediately after that I publish it on the ros topic. The problem is that despite being bytes if I try to do echo of the topic the values ​​become negative and this is problematic when converting from byte array to actual value reported by the sensor. I tried making (on the serial monitor of arduino) control prints but everything seems to be correct:

Packet ID:21 Packet length:2 Byte sequence: 134|1|
Packet ID:20 Packet length:2 Byte sequence: 134|3|
Packet ID:18 Packet length:2 Byte sequence: 240|0|
Packet ID:19 Packet length:2 Byte sequence: 134|1|
Packet ID:21 Packet length:2 Byte sequence: 134|1|
Packet ID:20 Packet length:2 Byte sequence: 137|3|
Packet ID:18 Packet length:2 Byte sequence: 240|0|

Than that when I run "rostopic echo /fromarduino":

id: 18
lenght: 2
value: [-16, 0]
timestamp: 62
---
id: 19
lenght: 2
value: [124, 1]
timestamp: 62
---
id: 21
lenght: 2
value: [124, 1]
timestamp: 62
---
id: 20
lenght: 2
value: [-112, 3]
timestamp: 62

Don't mind the timestamp that works correctly. I really hope some of you can help me. Thanks in advance.