I don't think this is exactly right, but I was overthinking it. I just saw that the user InternetTubes at lemmy.world got banned on both instances but InternetTubes at kbin.social has only been banned from lemmy.world, and just thought it hinted that an admin manually banned me across both instances the first time and forgot about this user the second time. Yet what's probably happening is that when a user is banned on the instance hosting the account, it gets automatically propagated across instances.