Closing as cannot reproduce, I haven't been able to get same issue either with XWiki 13.9 or 15.10.11. Here's the steps I used:
Logged in with Admin
Created a page MyPage
Created a page MyPage/SubPage
Created a page MyPage/SubPageXClass and added a String property in it
Created a page MyPage/SubPageXObject and added an object of type SubPageXClass
Tried to move MyPage to OtherPage
Clicked on "select all" when I get the questions about used xclass
In my case everything went well. I also tried to put the xobject in another hierarchy before performing the move, and the move worked properly too. Finally I also tried to select manually the page instead of clicking "select all", but without any difference.
Beat Burgener if it's still an issue for you, please reopen with indicating proper steps to reproduce the bug.
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