It's very simple to reproduce. Just place that on window:

TextEdit {
	selectByMouse: true
	x: 160
	y: 101
	text: "Да, но как же"
	readOnly: true
}

TextEdit {
	selectByMouse: true
	x: 160
	y: 131
	text: "Держи в уме"
	readOnly: true
}

And try to select all that text using mouse. I've attached the result.
text property contains words "Yes, but how" in russian, using cyrillic symbols. And something goes wrong with word "but" ("но") in 5.4.1, 5.4.2.

Second text edit contains "Держи в уме" ("Keep in mind" in Eniglish). That's a bit funny, but first text edit highlights text just fine in the latest 5.5.0, but this text edit shows the same error for letter "в" ("in").

I've also attached tiny project for that bug.