Setting the background-color of separators only works when separators are draggable

I'm using a stylesheet to change the size and color of separators between dock widgets and the central widget. Some of the docks have a fixed size (the separator cannot be dragged) and some are resizable. The stylesheet is changing the size of all separators but only changes the color of separators that can be dragged.

I've attached a program to reproduce the problem (main.cpp), a screenshot of what I expect to see (expected.png) and what I actually see (actual.png).

One of the members of the forum (SGaist) told me that he was able to reproduce the problem on Linux using the current dev branch of qtbase.