When you change the section of a subtree, only the parent is updated in Lucene index. The m_section_id isn't changed for children documents.

If sections are used as policy limit control in your website, SOLR will find (or not depending of the way you changed the section) documents but eZpublish will not allow user to see it.

So, you have a difference beetween result_count an real result to show to your user.

Steps to reproduce

1/ Test a request to find documents in a specific subtree

2/ Define a new section without read acces to anonymous

3/ Add a subtree to this section

4/ Test again the same request, you will find the same result_count but you won't see the documents.