Experiencing some inconsistencies with the presenceUpdate event.
All regarding a regular user that is not the bot.
The native Discord client properly reflects these events, and the old _presence_ event from v8 worked fine.
Users don't receive their own presence updates. I think this is what you're experiencing here.
I'm trying with a user that is unrelated to the bot, and the same seems to hold true for other users on the server. I have never seen an update where oldUser has a game and newUser is null since I moved to v9. Granted, my server only has a few people, but they have exited games enough that it's clearly not a coincidence.