https://doc.qt.io/qt-5/qcoreapplication.html#setLibraryPaths says:

Sets the list of directories to search when loading libraries to paths. All existing paths will be deleted and the path list will consist of the paths given in paths.

Which is not correct because the applications path is added:

#include <QApplication>
#include <QDebug>

int main(int argc, char *argv[])
{
    qputenv("QT_DEBUG_PLUGINS","1");
    QStringList sl;
    sl.append("/home/kapinter/Qt/5.15.1/gcc_64/lib");
    QCoreApplication::setLibraryPaths(sl);
    QApplication app(argc, argv);
    return app.exec();
}

and the log shows that it is still searching in the location where the application is.
The code is visible here void QCoreApplicationPrivate::init() :
https://code.qt.io/cgit/qt/qtbase.git/tree/src/corelib/kernel/qcoreapplication.cpp#n789