Amazon has posted an acknowledgment of the disappearing Kindle Direct Publishing books issue:
We're experiencing an issue that has made some eBooks unavailable in the Kindle store in different marketplaces. Our engineers are working to solve this issue as quickly as possible. We'll update this post to keep the Community informed once this issue has been resolved.
We appreciate your patience and understanding in the meantime.
Regards,
Kindle Direct Publishing Team
Bookmark this link to check for their updates.