Force replacing complete item on full champion with incomplete item

So I had 8 Rek'Sai with 3 complete items, thornmail + dragon tooth + Spear of Shojin.

I got to the carousel and found a Rek'Sai with giants belt. So i picked it up and it upgraded my Rek to lvl 3 but then it forced off my thornmail and replaced it with giants belt which was way worse against a lobby of rangers. I believe thornmail was my last item placed on Rek'Sai before upgrading him to 3 so I imagine that's where the problem lies? Not sure why something like this would be intentional.