Is this with the latest version of TAdvStringGrid?
I have retested this here but could not see an issue.
Test code applied to a default grid:
procedure TForm1.FormCreate(Sender: TObject);
begin
advstringgrid1.MouseActions.DisjunctRowSelect := true;
advstringgrid1.MouseActions.RowSelect:= true;
// tested with & without
advstringgrid1.MouseActions.DisjunctRowSelectNoCtrl := true;
advstringgrid1.RowCount := 500;
advstringgrid1.RandomFill(false);
advstringgrid1.SearchFooter.Visible := true;
end;