I don't know if this is a bug, or I missed some announcement, or this is a feature I just never noticed until now (I usually just gold everything, minus jewelry)... but I didn't think this is how it worked! I thought items got the stats depending on THEIR quality, not an average. This makes running purple jewelry even worse if it negatively degrades the other gold items in the same set
Yes, the set piece bonus of different qualities and levels averages out.
And it has been this way.
Technically purple jewelry does not make it worse. Since it averages out, it means the gold and purple gear each contribute appropriately to the final numbers when equipped. It would not very balanced if the averaging ignored lower level and quality gear.