In this case, it’s because you either input them as explicit rests with force duration or manually changed their vertical position using the Rest pos. property (which turns rests into explicit rests, so as to guarantee your edits are retained). If you select the minim/half note rest, you can see both the Rests and Force Duration icons in the toolbox are highlighted. Explicit rests don’t follow the Starts/Ends voice properties, because you’ve done something deliberate with them.
If you choose View > Note and Rest Colors > Implicit Rests, you can see that the quaver/eighth note rest in the upper voice becomes grey, whilst the lower rests do not. That tells you they’re explicit - you can then select them and press the Delete key on your keyboard to turn them back into implicit rests, at which point they will follow the Starts voice property should you activate it on that first note in the down-stem voice.