Here is a possible solution for solving this "missing check/mark" issue.
I implemented the CSS "indicator" solution I got from this qt form post which is problematic.
Sources of solution:
Doc page: here
Implementation: here
QPalette p = ui->checkBox->palette();
p.setColor(QPalette::Active, QPalette::Base, QColor(255, 255, 255));
p.setColor(QPalette::Button, QColor(255, 255, 255));
ui->checkBox->setAutoFillBackground(true);
ui->checkBox->setPalette(p);
QColor(255, 255, 255)
refers to your desired background color, possibly the same color as your window background, etc.
QPallete::Active, QPalette::base
refers to the active checkbox's background (the white box background)
QPalette::Button
refers to the "actual background" (behind the checkbox and the checkbox text)
Hope this helps!