BUG #18114: FULL JOIN is replaced by LEFT JOIN in plan

The following bug has been logged on the website:

Bug reference: 18114
Logged by: crvv
Email address: crvv(dot)mail(at)gmail(dot)com
PostgreSQL version: 16.0
Operating system: Linux
Description:

SELECT * FROM (VALUES (1)) AS t(id)
CROSS JOIN unnest('{1,2}'::int[]) AS a(x)
FULL JOIN unnest('{2,3}'::int[]) AS b(x) ON a.x = b.x
WHERE t.id = 1

Execute this SQL, I get the result
id | x | x
----+---+---
1 | 1 |
1 | 2 | 2

My expection is
id | x | x
----+---+---
1 | 1 |
1 | 2 | 2
| | 3

The query plan is
QUERY PLAN
-------------------------------------------------------------------
Nested Loop Left Join (cost=0.01..0.08 rows=1 width=12)
Join Filter: (a.x = b.x)
-> Function Scan on unnest a (cost=0.00..0.03 rows=1 width=8)
Filter: (1 = 1)
-> Function Scan on unnest b (cost=0.00..0.02 rows=2 width=4)

So I think the FULL JOIN is replaced by LEFT JOIN.

The following SQL statements both give me the expected result.
SELECT * FROM (VALUES (1)) AS t(id)
CROSS JOIN unnest('{1,2}'::int[]) AS a(x)
FULL JOIN unnest('{2,3}'::int[]) AS b(x) ON a.x = b.x;

SELECT * FROM (VALUES (1)) AS t(id), unnest('{1,2}'::int[]) AS a(x)
FULL JOIN unnest('{2,3}'::int[]) AS b(x) ON a.x = b.x
WHERE t.id = 1;

I can reproduce on PostgreSQL 16.0, 15.4 and 14.9.