When ArtRage drops in to the background it attempts to save the current live canvas to a temp file so that it can be reloaded later (iOS requires that we do this, the app basically unloads its contents when it drops in to the background). This process is generally pretty quick but in some cases with a large canvas on a device that has a lot of stuff going on in the background it can take a few seconds. When you return to the app, the last backup file is reloaded. If you came back in to the app really quickly it's possible that the app hadn't had time to save the old canvas.

I've not seen this happen with smaller canvases on modern devices, and we do actually have a warning that tells you when this has happened, but you may have disabled that warning if you've seen it before.

So - the short answer: This shouldn't happen, but if you switch apps really quickly, it might. I'm going to see if I can think my way to a solution for that!