I had this same issue and after trying many things I noticed that the ordering of the AutoLoad property matters. Here I am creating a QDSCV and eventually assigning it to a RadTreeListView and a RadDataPager in XAML with an MVVM approach.
When placed before the PageSize (see below), it ignored the PageSize on the first load and loaded all rows. When I placed the PageSize above the AutoLoad, I get my page size of 100 on the first load. Bug?
var query = _context.GetSearchDetailMembershipIndexedByMembershipViewsQuery().OrderBy(m => m.Name);
// create the QDSCV, using the context and the query
var view =
view.PageSize = 100;