JSON encoded timestamp value does not always match non-JSON encoded value

$ sudo docker pull cassandra:4.1.4
$ sudo docker create --name cass cassandra:4.1.4
$ sudo docker start cass
$ # wait for the Cassandra instance becomes ready
$ sudo docker exec -ti cass cqlsh
Connected to Test Cluster at 127.0.0.1:9042
[cqlsh 6.1.0 | Cassandra 4.1.4 | CQL spec 3.4.6 | Native protocol v5]
Use HELP for help.
cqlsh> create keyspace test WITH replication = {'class': 'SimpleStrategy', 'replication_factor': 1};
cqlsh> use test;
cqlsh:test> create table tbl (id int, ts timestamp, primary key (id));
cqlsh:test> insert into tbl (id, ts) values (1, -13767019200000);
cqlsh:test> select tounixtimestamp(ts), ts, tojson(ts) from tbl where id=1;
 system.tounixtimestamp(ts) | ts                              | system.tojson(ts)
----------------------------+---------------------------------+----------------------------
            -13767019200000 | 1533-09-28 12:00:00.000000+0000 | "1533-09-18 12:00:00.000Z"
(1 rows)
cqlsh:test> select json * from tbl where id=1;
 [json]
---------------------------------------------
 {"id": 1, "ts": "1533-09-18 12:00:00.000Z"}
(1 rows)