BUG #19454: PL/pgSQL mishandling jsonb attribute reference

The following bug has been logged on the website:

Bug reference: 19454
Logged by: CN Liou
Email address: ma(dot)sao(at)msa(dot)hinet(dot)net
PostgreSQL version: 18.3
Operating system: Linux Debian Bookworm
Description:

It appears the PL/pgSQL assignment operator := fails to maintain the
stability of a jsonb attribute reference (from a function result) during a
self-concatenation operation, whereas a SELECT wrapper forces correct
materialization.

DO $$
DECLARE
v_payload JSONB := '[]'::jsonb;
tj JSONB;
BEGIN
tj := '{"delta": [["1221", "1221", "TWD", 577.82, {"tags":
[]}]]}'::jsonb;
RAISE NOTICE 'Before: %, Delta is Null: %', (v_payload IS NULL),
(tj->'delta' IS NULL);
v_payload := v_payload || tj->'delta'; -- The problematic line
--v_payload := (SELECT v_payload || (tj->'delta')); --This avoids
the issue.
RAISE NOTICE 'After: %', (v_payload IS NULL);
END $$;