Partition key causes problem for volatile target list query

I have found an odd behavior --- a query in the target list that assigns
to a partitioned column causes queries that would normally be volatile
to return always zero.

In the first query, no partitioning is used:

d1 | d2
----+----
1 | 0
2 | 0
2 | 1
1 | 0
1 | 2
1 | 2
1 | 0
0 | 2
2 | 0
2 | 2

In the next query, 'd1' is a partition key and it gets a constant value
of zero for all rows:

d1 | d2
----+----
--> 0 | 1
--> 0 | 2
0 | 2
0 | 1
0 | 2
0 | 1
0 | 2
0 | 2
0 | 2
0 | 2

The self-contained query is attached. The value is _always_ zero, which
suggests random() is not being called; calling setseed() does not
change that. If I change "SELECT x" with "SELECT 2", the "2" is used.
I see this behavior back to PG 11.

--
Bruce Momjian <bruce(at)momjian(dot)us> https://momjian.us
EDB https://enterprisedb.com

Embrace your flaws. They make you human, rather than perfect,
which you will never be.