Firtst of all I must admit that this is double bug: one in QtCsv library (independent of Qt) and in Qt QString class.
It seems that somewhere deep in the QString::operator==() is bug. It not appear in "normal code". Due to bug in QtCsv library it try to compare end of string "\0" with "\'"" (double quote - one character string). This is not happen in Qt 5.9.7.
To reproduce this bug you have to download attached minimal working example, unpack, open in Qt Creator and chose Qt 5.12 and simply hit F5. Then it will build QtCsv and example program, and it will run it in debuger.
QtCsv bug is reported here: QtCsv bug report