Thanks a lot for your quick replies!
@SGaist
I tried to access the individual character using QString::at(). Both at(0) and at(1) returned something that was printed as square in a QLineEdit. When I pasted these characters into the text editor of this forum, a code was shown in the square. After removing the space between these two characters, they were fused to the single one described the link in my first post.
@Chris Kawa
QString::size() returned 2. I encountered this while parsing the kanjidic2.xml file, which contains information on letters of the Japanese language. The QString causing this problem was initialized using QXmlStreamReader::readElementText(). I assumed the string size to be 1, as the respective xml field contains only a single literal. Your second post helped me a lot, thank you.
I thought that a single QChar was able to represent any unicode character, but the first line of its documentation already restricts it to UTF-16.
I think that the documentation of QString::size() and QString::at() should point to this issue somehow. If I would have assumed my QString's size to be always one (I was extracting single literals after all) and hadn't verified this, it would have caused an extremely well hidden bug in my program.