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 versions and don't already have a backup, it's highly recommended to do a backup of your database as soon as possible, so that you don't loose more content from now on.
A fix is under way, and 15.10.10 and a 16.4.0 are going to be released in the coming days (most probably at the beginning of next week).
In the meantime, or if you just can't upgrade, 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-15.10.9.jar and you should keep the same file name to avoid problem during next apt upgrade) with one of the following patched JAR files:
We apologize for the problems this may cause.
