Please disregard my comment about not able to reproduce. I misunderstood the issue late last night. I can now reproduce.
Clicking on the "More info..." link should open a new "About Qt" dialog window instead of changing the contents of that HTML view widget. Instead it seems to switch the HTML view to a blank page. Maybe related to the switch from QWebView to QWebEngineView. It seems that the BtAboutDialog::linkClicked slot does not get called at all.