Entity `RETURN DISTINCT` followed by `UNION` fails with `type with OID ... does not exist`

[YGY-001]

Reproduction

LOAD 'age';
SET search_path = ag_catalog, public;
SELECT ag_catalog.create_graph('age_issue_case18');

SELECT *
FROM ag_catalog.cypher('age_issue_case18', $$
CREATE (n {id: 1})
$$) AS (v agtype);

SELECT *
FROM ag_catalog.cypher('age_issue_case18', $$
MATCH (n)
RETURN DISTINCT n AS value
UNION ALL
RETURN null AS value
$$) AS (value agtype);

Expected

The query returns the matched vertex and one null row. UNION ALL permits
both rows, and null is a valid value for the same result column.

Actual

ERROR: type with OID 4026532032 does not exist

The numeric OID varies with the fresh database. It is generated internally;
there is no OID or type name in the Cypher input. The failure reproduces on a
fresh AGE 1.8.0 / PostgreSQL 18.6 graph.

Controls

The following queries succeed on the same graph:

MATCH (n)
RETURN n AS value
UNION ALL
RETURN null AS value

RETURN 1 AS value
UNION ALL
RETURN null AS value

The failure therefore requires the entity branch's RETURN DISTINCT path;
it is not a user-supplied missing-type or missing-OID error. The original
audit Cases 951, 1167, and 1888 are deduplicated under this one family. Case
1167 uses UNION ALL; Cases 951 and 1888 use the default UNION spelling.

Environment

Apache AGE 1.8.0 on PostgreSQL 18.6, Docker image
apache/age:release_PG18_1.8.0.