The issue is not reproduced on XWiki 14.10.6 and older versions, but occurs starting with XWiki 14.10.7.

This is the list of changes in 14.10.7 compared to 14.10.6: https://www.xwiki.org/xwiki/bin/view/ReleaseNotes/Data/XWiki/14.10.7/

The one item that seems directly related is "Upgrade to jstree 3.3.15" (XWIKI-20627).