Perhaps your S-Pen was locked by another task, which could explain why the game reacted the way it did. If the Pen was locked by another task, it would expect to see that task open/in use/active, and then finds another app in use, it might cause unexpected behavior. Again it would be so much easier if there was a clear way to force stop the S-Pen and/or close the S-Pen app. You might think that restarting the phone would be enough but have you noticed that background apps repopulate even after a restart? I get it, those were the last things running and they may be technically closed/stopped once the device restarts but somehow they are still there. Perhaps in this same way even with restarts the S-Pen is still locked into this phantom task.
Again since we're not able to go to a specific place to clear the cache or force stop the app, perhaps getting an app like Cclean and doing a full phone cache clear might help push this task out.
Out of curiosity what did you do when that popped up? Did you discard it and not save as a draft? Have you gone into that email account and discarded it since? With this being the only clue so far, perhaps clearing the cache on your email app alone, to see what the result is...?