After a revert on en:Barack_Obama, the pageimage property as returned by the action API disappeared. A subsequent minor (not pageimage related) edit brought it back within seconds, and also quickly updated the derived page summary response in the REST API.
Given the quick recovery, it seems unlikely that the pageimage property update was delayed for over an hour by job queue backlogs. The more likely explanation seems to be that the pageimage update was completely missed.
I have not yet tried to reproduce this issue by reverting a page multiple times. It might or might not be specific to reverts. It could also be caused by a random job being lost in the job queue.