Currently, the vectorized hash aggregator doesn't maintain the partial ordering if it has to spill to disk. Consider the following logic test which will fail on fakedist-disk config:

statement ok
create table ab (a int, b int, index(a) storing (b));
insert into ab values (1,1),(3,3),(2,2),(5,5),(0,0),(1,1);

query III
select a, b, count(*) from ab group by a,b order by a
----
0  0  1
1  1  2
2  2  1
3  3  1
5  5  1

The issue is present only on 21.1 since before this release we didn't have the disk spilling support. There are several possible ways to mitigate this problem, and as the first step I will look into supporting the partial ordering by the external hash aggregator.