I tested your code on PostgreSQL using C++ and got correct results (I used a query that returns a single record, looks like you did the same). May be there is a bug in SqlLite driver or in PyQt4?

Test program
Qt Code: 
  1. int main(int argc, char* argv[])
  2. {
  3.   QCoreApplication app(argc, argv);
  4.  
  5.   QSqlDatabase db = QSqlDatabase::addDatabase("QPSQL");
  6.   db.setDatabaseName("pm");
  7.  
  8.   bool ok = db.open();
  9.   if(! ok) {
  10.     qDebug() << db.lastError().text();
  11.     return 1;
  12.   }
  13.  
  14.   QSqlQuery query;
  15.   if(query.exec("select * from pm.portfolio where id = 1")) {
  16.     qDebug() <<  "is_active" << query.isActive();
  17.     qDebug() <<  "is_select" << query.isSelect();
  18.     qDebug() <<  "is_valid" << query.isValid();
  19.  
  20.     qDebug() <<  "";
  21.     qDebug() <<  "seek:" << query.seek(0);
  22.     qDebug() <<  "is_valid" << query.isValid();
  23.  
  24.     qDebug() <<  "";
  25.     qDebug() <<  "seek:" << query.seek(0);
  26.     qDebug() <<  "is_valid" << query.isValid();
  27.  
  28.     qDebug() <<  "";
  29.     qDebug() <<  "next" << query.next();
  30.     qDebug() <<  "is_valid" << query.isValid();
  31.  
  32.     qDebug() <<  "";
  33.     qDebug() <<  "seek:" << query.seek(0);
  34.     qDebug() <<  "is_valid" << query.isValid();
  35.   }
  36.   else {
  37.     qDebug() << query.lastError();
  38.   }
  39. }
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Result
is_active true
is_select true
is_valid false

seek: true
is_valid true

seek: true
is_valid true

next false
is_valid false

seek: true
is_valid true