Unexpected result with aggregate inside subquery

CREATE TABLE TBL1(ID INT PRIMARY KEY, VAL int);
insert into TBL1 values (1, 1), (2, 2);
select (select(sum(VAL))) from TBL1; <-- await 3 as a result

1
2

select (select(sum(I))) from bar;

 EnumerableCalc(expr#0..2=[{inputs}], EXPR$0=[$t2])
   EnumerableCorrelate(correlation=[$cor0], joinType=[left], requiredColumns=[{0}])
     EnumerableTableScan(table=[[BLANK, BAR]])
     EnumerableAggregate(group=[{}], EXPR$0=[SUM($0)])
       EnumerableCalc(expr#0=[{inputs}], expr#1=[$cor0], expr#2=[$t1.I], $f0=[$t2])
         EnumerableValues(tuples=[[{ 0 }]])