I had a similar issue, but I didn't even got my rewards.
I had opened the arena just to check if I got my play points for the Mythic limited, (one day before I was on mythic 777). I opened the app, showed mythic tier one and showed new rank platinum (not sure which). I closed straight away the phone. This morning I opened the app, showed showed the platinum and one second later, appeared my new rank as silver.
I don't care too much for the boosters that I lost, but I'd like my play points.
I had a similar issue, but I didn't even got my rewards.
I had opened the arena just to check if I got my play points for the Mythic limited, (one day before I was on mythic 777). I opened the app, showed mythic tier one and showed new rank platinum (not sure which). I closed straight away the phone. This morning I opened the app, showed showed the platinum and one second later, appeared my new rank as silver.
I don't care too much for the boosters that I lost, but I'd like my play points.