This is a regression caused by the problem described originally in Bug #44056, which has been closed meanwhile and not been reopened. The original problem does not exist anymore, but has been exchanged for another one, so maybe a new bug should be filed about this.

The problem is that, if the Fusion style is used on Linux, QGroupBoxes without titles have much more space on top than would be needed. Using Windows or macOS, this does not happen. There, title-less QGroupBoxes are rendered as expected.

I'll attach (again) a real-life screenshot showing the issue (left one: old wrong rendering before Qt 5.14, right one: new wrong one now) and a minimal example producing it.

Thanks in advace for (eventually) fixing this!