The top answer has effectively answered your question about how Walpurgisnacht's appearance is predicted in later timelines. However, its appearance is still known in advance without Homura's intervention.
- In Madoka Magica episode 10, Mami is actively preparing for Walpurgisnacht's arrival.
- In Kazumi Magica chapter 22, Kyubey briefly mistakes Kanna's amalgamate witch for Walpurgisnacht, implying that it could suddenly appear in a similar manner.
- In The Different Story chapter 2, Kyouko tells Mami rumors she's heard about a superdreadnought witch called Walpurgisnacht, which is notable since this long predates Walpurgisnacht's arrival in the anime.
- In Magia Record, the Magius' plan revolves around luring Walpurgisnacht to Kamihama as the power source needed for the Automatic Purification System to extend around the globe.
Most notably, however, Walpurgisnacht does not drop a grief seed. In the timeline where it is defeated by Madoka and Homura alone, the only grief seed they have access to is Oktavia's.
This can't be attributed to the annihilation of its soul as in Magia Record, as neither Madoka nor Homura have a power capable of such like Ren does. And Portable does confirm it has a Grief Seed.
So where does its Grief Seed go, if it has one but never drops it even when seemingly defeated? Well, what if it doesn't die when it's killed? This would readily explain how it's well-known in advance; it's never truly killed, but simply exits reality and waits. It has been defeated before, by magical girls who would warn their younger teammates of the witch that tears open the day, and it would be defeated again if the timeline hadn't consistently been reset after its appearance in the anime.
Further supporting evidence, which is less reliable due to being pulled from unreleased materials:
The Rebellion sequel's title, Walpurgis Revolution, implies that the witch is somehow capable of persisting past two true resets, even though logically the Law of Cycles should immediately destroy it once it appears.
A witch exclusive to Portable, Itzli, is specifically noted in the lore as only appearing after Walpurgisnacht's defeat. Like the character introduced by Scene/Zero, Aki Mabayu, Itzli has a film-splicing motif, and could thus potentially have powers over editing herself into reality if she is in fact a part of the Walpurgisnacht amalgamate. Supporting this is Mabayu's suspiciously Walpurgisnacht-like appearance: