I use sumatra and TeXstudio 2.6.2 (SVN 4110M).
When I launch an instance of the viewer by F3 or the icon, sumatra appears fine, when ctrl-alt to focus on texstudio, and press f3/click on the icon anew, sumatra does not appear, nothing is happening in the window "message" at the bottom of texstudio.
All this regardless of the enabling of the option "do not launch a new instance of the viewer etc."
and/or put reuse-instance in the cmd line of sumatra.
It did work for previous versions of texstudio.