I have this theory that chapter 686 is Orihime's fault. Since she was always jealous of Ichigo's bond with Rukia and was never keen of him going out and doing soul reaper duties because it was too dangerous, she used her powers to reject the entire events of the manga. That's my explanation why the Soul Society had reverted to the way it was before Ichigo came along and why Ichigo is rotting in the human world, seemingly not learning a thing.
Omg that made me laugh. XD As much as IH would want their goddess uwu to pull that kind of badassery, she isn’t capable of that level of rejection since she couldn’t counter Yhwach’s power of future and reject damage to Ichigo’s sword, which was a very limited area, without help from Tsukishima.
But if we are talking about theories, let me give you an interesting apocalypse theory I read about bleach. It basically said the ending doesn’t actually exist. Aizen and Ichigo tried to save the world but it was too late. Since Yhwach was the Soul king the world ended after he succumbed. Aizen tried to take over but by the time he managed to become soul king the world had already been destroyed so the last chapter is a hypnosis casted by Aizen (like eternal tsukiyomi in naruto) to create a perpetual state of dreams and bleach world as we know it no longer exists.
Of course I don’t fully agree with this theory but it was interesting to read anyway.