Qt 5.14 introduced smarts to allow for QStyleOptions control DPI scaling. However, there is an issue where DPI scaling is wrong if this isn't provided. In QStyleHelpers, if the options aren't provided a fixed value of qstyleBaseDpi is used.
See "Widget style: Use per-screen DPI in QStyleHelper::dpiScaled()" d603ee689f0e3fdcfa3230b3d75cdce6c5af05c1 https://codereview.qt-project.org/c/qt/qtbase/+/271429 .
Previously the default was DPI scaled... albeit by the main monitor's logical DPI.
This is a functional change that has the right idea but it really means that options must be provided everywhere for a visually consistent experience. However, this isn't the case as many places internally like QFusion aren't consistent.
As a work-around I created a proxy style that created a default style object. This only works in a single-DPI case, though.
A recommended real fix would be patching all usage points to include options and perhaps doing a better default in the no-options case (which probably should be a logged as a warning as well).
NOTE: I see some other bugs claiming this was fixed but I think the root issue still exists.