generate_series regression 9.6->10

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.