src/Pure/Thy/sessions.scala

           val other_name = info.name + "_requirements(" + ancestor.get + ")"
           (other_name,
             List(
               make_info(info.options,
                 dir_selected = false,
-                dir = info.dir,
+                dir = Path.explode("$ISABELLE_TMP_PREFIX"),
                 chapter = info.chapter,
                 Session_Entry(
                   pos = info.pos,
                   name = other_name,
                   groups = info.groups,