I wouldn't assume that.
QLatin1String latinStr("Adiós");
qDebug() << latinStr << newStr;
Gives:
"Adi\u00C3\u00B3s"
A workaround that you can try:
QString unicodeStr = "some Uniocode හාහාහා %1";
QString newStr = unicodeStr.arg(QString::fromUtf8(latinStr.data()));
qDebug() << newStr;