Yeah, happens to me sometimes. If you kill the app and load it up again, then they SHOULD reappear again, in all their pixelated glory. I believe if you leave the area before you open the app or don't start it within five minutes, then they'll go.
This is the thing, I could restart the app three times in a row, it would not solve the issue.
I'd say resetting the app solves it like 25% of the time or less.