As found in QTBUG-103043:

The following code fails for some strings with specific unicode characters, e.g. "Straße" and "\uFB03" (U+FB03 LATIN SMALL LIGATURE FFI):

QString hayStack = u"dhwqdwqdwqjdwqhdl" + mixedCase + u"dsadasdads";
QVERIFY(hayStack.contains(mixedCase.toUpper(), Qt::CaseInsensitive));

The following code works for "\uFB03" but fails for "Straße":

QVERIFY(mixedCase.normalized(QString::NormalizationForm_KC).compare(mixedCase.toUpper(), Qt::CaseInsensitive) == 0);

This is similar to QTBUG-135301 but the Fix is probably more difficult since Boyer-More has to account for possible variations in string lengths.