[Sliding sync] to-device messages suddenly stop getting delivered on one device #20355

This comes from a rageshake from a private server, so unfortunately, information is limited. The user reports that messages are being delivered normally to one of their devices (Element Desktop, so using legacy sync), but on another device (Element X iOS, so using sliding sync), it just says "waiting for message" (i.e. it isn't getting the megolm keys to decrypt). Looking at the logs indicates that the sync token seems to be doing strange things, such as sections of the token decreasing rather than increasing (see the rageshake issue for details), and we're not sure if that's normal or not.