On my level up to 7 I got two cases, one without any “can drop rare stuff” text and one with that text.
But after checking my inventory I only had one case.
I opened the only case I had, and got a lead SMG proxy card.
I traded 3 of my lead cards (including my new proxy card) for a iron Aura one and the proxy one didn’t disappeared.
After relaunching the game the card did disappeared.