Editable QComboBox by default has clearly visible border lines. In the case that parent widget has a background color set & the application uses "fusion" style the parent widget background color will be partially drawn inside the combobox edit area. For reference, see sample.png.
The method of applying the background color seems irrelevant as the result will look the same regardless of whether the background color was applied with stylesheet or qpalette.
This is a regression from Qt 5.11.
Steps to reproduce:
Run the attached project.