When the parent value of the old parent-child relation is set upon the move of a page (
XWIKI-13493 ), apparently the parent value is set as the fully prefixed reference of the parent, including wiki name.
In order to match the backwards compatibility need (as expressed by
XWIKI-13493), it should not be set fully prefixed, as it was not the case before (when parents were set manually for pages and automatically upon rename of the parent page).
None of the code that is querying pages by the parent name (old style) is querying for prefixed value, as it was never stored prefixed before.
Also, a sort-of principle of XWiki storage is that the name of a database is never stored inside that same database, as much as possible (excepting content of documents where users could write whatever they want) in order to increase flexibility of data move.