setFont() Applying Everything except Size Changes
You've set a fixed font size for the QLabels in the ui therefore when you change the application's global font size, it won't affect those.
<font>
<pointsize>10</pointsize>
</font>
Changing the application's font size sets a default size for the widgets. If you set the font size of one specific widget, it will overwrite the default one (this is what happened in your case).
If you want to change the font size, set the widget's font, like:
auto lbl = ui->label_2;
auto font = lbl->font();
font.setPointSize(2);
lbl->setFont(font);