Unfortunately, there are currently several Qt issues on the Big Sur without proper solution.
There are issues in the styling https://bugreports.qt.io/browse/QTBUG-86513,
and also there are issues on the controls with Big Sur https://bugreports.qt.io/browse/QTBUG-85546
Not sure what caused this mess but obviously Big Sur changed a lot of underlying logic and Qt didn't react quickly enough ;-(.
Currently, there is a lot of Qt GUI issues that are waiting for fixes. Right now, we are not able to fix these invalid behaviors ;-(.
We will closely watch the progress on these bugs and release new versions with upcoming fixes as soon as possible.
Regarding color chooser, we will try to replace it with another component as a temporary solution.