A regression related to the manipulation of the history of a page was introduced in XWiki 15.10.8 and 16.3.0, and it also impacts 15.10.9 and 16.3.1: if you move a page which contains an attachment, the history of the page will be lost without any way to recover it (unless you have your own backup, of course).
If you run one of these version, it's highly recommended to do a backup of your database ASAP so that you don't lose more content from now on.
A fix is under way and a 15.10.10 and a 16.4.0 are going to be released in the coming days (most probably beginning of next week).
In the meantime or if you just can't upgrade for some reason, you can replace the file xwiki-platform-legacy-oldcore-<version>.jar located in <xwiki home>/WEB-INF/lib (for example, if you use the Debian package of XWiki 15.10.9, the complete path would be /usr/lib/xwiki/WEB-INF/lib/xwiki-platform-legacy-oldcore-16.3.1.jar and you should keep the same file name to avoid problem during next apt upgrade) with one of the following patch JARs:
- 15.10.8:
- 15.10.9:
- 16.3.1:
