QFrame with FusionStyle, WindowsStyle (and other styles), or anything that calls qDrawPlainRect() has painting artifacts in the left and top, when in scaled/HighDpi mode.
The lines are too thin, perhaps clipped. The reason seems to be the typical off-by-one of code that was written for normal resolution usinf the integer QPainter-API.
Some adjustments like the following one fix the issue. However, that needs to be thoroughly tested, especially to prevent regressions in the unscaled mode.