QML Canvas improperly rendered with QT_SCALE_FACTOR set - qt

Question

I have a Qt/QML program that runs on Ubuntu and on a Raspberry Pi 3B+. The program runs fine on both platforms however the Pi has a high DPI screen so I set a QT_SCALE_FACTOR to 1.3 on that device to make controls appear the right size. It works perfectly except for one thing: indicators on ComboBox and SpinBox controls are drawn the same size as if no QT_SCALE_FACTOR was set, and the rest of the rectangle they would fill if the QT_SCALE_FACTOR was applied is filled with random pixels. Setting a QT_SCALE_FACTOR on Ubuntu works without any issues.
I tried replacing the indicators with a custom one but I get the same result.
Leaving QT_SCALE_FACTOR unset produces correctly rendered indicators.
This is with Qt 5.13.2 and QtQuick.Controls 2.5 on Linux pi3 4.19.75-v7+ #1270 SMP Tue Sep 24 18:45:11 BST 2019 armv7l GNU/Linux
The pictures below illustrate the behavior with QT_SCALE_FACTOR=2:
This is the code for the custom indicator in the first picture:
ComboBox {
id: cb
...
width: 200
...
indicator: Canvas {
id: canvas
x: cb.width - width - cb.rightPadding
y: cb.topPadding + (cb.availableHeight - height) / 2
width: 12*2
height: 8*2
contextType: "2d"
Connections {
target: cb
function onPressedChanged() { canvas.requestPaint(); }
}
onPaint: {
var context = getContext("2d");
context.reset();
context.beginPath();
context.moveTo(0, 0);
context.lineTo(width, 0);
context.lineTo(width / 2, height);
context.closePath();
context.fillStyle = cb.pressed ? "#17a81a" : "#21be2b";
context.fill();
}
}