I use the sample code for QWebEngineView available at https://doc.qt.io/qt-5/qtwebengine-webenginewidgets-minimal-example.html

On Ubuntu 20.04.3 LTS with Gnome, if I minimize the sample application window, then un-minimize the windows I only get an empty grey window. See attached minmax.png picture.

I can get the correct web page content (redraw) by resizing the window. See resize.png picture.

i also tested on Ubuntu 21.04 using Qt 5.15.6 and I still reproduce the problem.

But I then installed KDE on my Ubuntu 20.04 and this time I do NOT have the issue using the same binary as I used on GNOME.

Something is special when GNOME (3.36.8) is the desktop environment. The problem is present with both GNOME+X11 and GNOME+Wayland.

One way I found to avoid the problem is to remove the minimize button from the window. But that is not a correct fix.

I attach my source code.