Combining Qt::ExpandedClientAreaHint with Qt::CustomizeWindowHint breaks mouse event handling

When I set the Qt::ExpandedClientAreaHint and Qt::CustomizeWindowHint to create a custom header for the window, it no longer handles mouse events. Keyboard input works fine, I can navigate through the input widgets and type into line edits etc. But I can't hover or click on anything.

Removing either Qt::ExpandedClientAreaHint or Qt::CustomizeWindowHint brings back mouse events.