8 of 12 Copilot notebooks suddenly disappeared

1. I have the same issue and I'm still not sure what caused it and why I was able to resolve it. After the first one disappeared, I tried to access the remaining ones and they disappeared as soon as I opened them and returned to the list.
2. They occasionally reappeared and I was able to keep those that I added to my favorites. If I didn't, they disappeared once again. This way I was able to keep one for some time and hoped that the others would appear again.
3. There seems to be no official way to resolve this, I'm not sure if there is any way to restore vanished chat histories, however today I realized that my chat history contained chats that I started in said vanished notebooks and prompted something like "A quick off-topic question: As far as I recall, I started this chat in a notebook, but I can no longer find it. Do you still have this context?" some chats said yes, and even listed IDs, some said that they don't know directly, but have other chats as reference. But prompting this somehow triggered the backend to restore the notebook each chat was related to. I restored 5 notebooks this way.
4. I don't know what Microsoft would call that, but it's definitely not a client issue, since those notebooks weren't visible in any of my clients. I tried Android, Web, Desktop and even OneNote before and after restoring.

I hope this helps in any way and I wish you good luck.