Strange color updates of custom quick items in the QML scene

Simple test case: two custom quick items implementing a rectangle (by using an out-of-the-box QSGSimpleRectNode) are placed on a Flickable surface. When trying to change the color of the first rectangle (on a timer event) its color won't change until I drag the surface. When trying to change the color of the second rectangle in the same way, both rectangles change color. However, the first rectangle gets back to its original correct color when I drag the surface.