The behaviour of generate_series seems to have changed a little, at least in conjunction w/ CTEs. Under 9.6 (and prior) this query returns 2127 rows, with no nulls:
with
ij as ( select i, j from generate_series(1, 10) i, generate_series(1, 10) j),
iijj as (select generate_series(1, i) as a, generate_series(1, j) b from ij)
select a, b from iijj;
Under 10, it returns only 715 rows, with many nulls.