Rendering dynamic text with big font fails

In the code that displays the simple number "20" as shown below, a display error occurs depending on the situation.
Using the Orbitron font
runtimeEditable: true
font.pixelSize is 153 or lower: OK
font.pixelSize is 154 or higher: NOK

SafeText {
          id: textItem
          objectName: "textItem"
          height: 200
          width: 600
          color: "black"
          verticalAlignment: Text.AlignVCenter
          horizontalAlignment: Text.AlignHCenter
          font.pixelSize: 154
          font.family: "Orbitron"
          fillColor: "white"
          text: "20"
          runtimeEditable: true
}

Only the first letter is rendered if the size is bigger than 153.

The root cause is that we have reserved signed int8 for the advance variable and the value does not match to the variable.