Weird canvas behavior in QT/QML - qt

Question

I have a strange issue in QML when trying to draw relatively simple graphics.
I'm using PyQt 5 with QT 5.11 and Ubuntu 18.04.
Since the graphics part I'm writing is mostly stationnary (changes about each 1 secs), I found out canvas would be a convenient way to draw graphics without using QPainter. But it's been noting but a nightmare with fonts.
For example, when drawing this simple QML component:
import QtQuick 2.11
import QtQuick.Controls 2.4
import QtQuick.Layouts 1.11
import QtGraphicalEffects 1.0
Rectangle {
id: cont_rect
Button {
text: "Repaint"
onClicked: {
cv.clearcv()
cv.requestPaint()
}
}
property string cvfont: "13px Ubuntu"
height: 80
width: 300
color: "#132931"
Canvas {
anchors.fill: parent
antialiasing: true
contextType: '2d'
id: cv
width: cont_rect.width
height: cont_rect.height
function clearcv() {
var ctx = cv.context
ctx.clearRect(0, 0, width, height)
ctx.fillStyle = "black"
}
function drawStateLine() {
var ctx = (cv.context == null) ? getContext("2d", {alpha: false}) : cv.context
var x = width/2, y = height - 10
var txt_angle = -45 * 2*Math.PI / 360
ctx.save()
ctx.translate(x,y)
ctx.rotate(txt_angle)
ctx.fillStyle = "royalblue"
ctx.textAlign = "left"
ctx.font = cvfont
ctx.textBaseline = "middle"
ctx.globalAlpha = 1
ctx.fillText("Detected", 0, 0)
ctx.restore()
}
onPaint: {
drawStateLine();
}
}
}
I end up with extremely weird graphics behavior.
The FIRST time my component is rendered, I have good looking text:
Then, when I click my specially crafted button (for the sake of isolating the problem down to a simple component), I have:
Basically, whenever I am trying to repaint my damn canvas, first clearing it (I also tried ctx.reset(), ctx.fillRect with my background color, etc.), I end up with font rendering that is much less readable and that bothers me.
Is there anyone here who has an idea on how to avoid this?
I don't know if it's useful, but I use a 27in display with 1080p resolution.
Thanks!