I dont understand what column ORA-01779 is refering too - sql

Question

I have a table with POI, you can have multiple POI on each city.
SQL DEMO
CREATE TABLE POI
("poi_id" int GENERATED BY DEFAULT AS IDENTITY,
"city_id" int,
PRIMARY KEY("poi_id")
);
Now there were some changes on the cities polygons and now have to reassign some POI
CREATE TABLE newCities
("city_id" int, "new_city_id" int)
;
DATA
INSERT ALL
INTO POI ("poi_id", "city_id")
VALUES (10, 1)
INTO POI ("poi_id", "city_id")
VALUES (11, 1)
INTO POI ("poi_id", "city_id")
VALUES (12, 2)
INTO POI ("poi_id", "city_id")
VALUES (13, 2)
INTO POI ("poi_id", "city_id")
VALUES (14, 5)
SELECT * FROM dual
;
INSERT ALL
INTO newCities ("city_id", "new_city_id")
VALUES (1, 100)
INTO newCities ("city_id", "new_city_id")
VALUES (2, 200)
INTO newCities ("city_id", "new_city_id")
VALUES (3, 200)
SELECT * FROM dual
;
When I do a JOIN:
SELECT *
FROM poi p
JOIN newCities nc
ON p."city_id" = nc."city_id";
OUTPUT
+--------+---------+---------+-------------+
| poi_id | city_id | city_id | new_city_id |
+--------+---------+---------+-------------+
| 10 | 1 | 1 | 100 |
| 11 | 1 | 1 | 100 |
| 12 | 2 | 2 | 200 |
| 13 | 2 | 2 | 200 |
+--------+---------+---------+-------------+
But when I try to do the update but got the error:
ORA-01779: cannot modify a column which maps to a non key-preserved table
UPDATE (
SELECT p.*, nc."new_city_id"
FROM poi p
JOIN newCities nc
ON p."city_id" = nc."city_id"
) t
SET t."city_id" = t."new_city_id";
I know city_id isn't a PK but the row is match with a row including a PK. So why isn't working?
I know I can do a sub query to get the value:
UPDATE poi p
SET "city_id" = COALESCE((SELECT "new_city_id"
FROM newCities c
WHERE c."city_id" = p."city_id")
, p."city_id");
But still want to know what cases the UPDATE JOIN would work because looks like only can work to update the PK:

Answer 1

After doing the sample test decide try creating a PK on the second table and works:
SQL DEMO
CREATE TABLE newCities
("city_id" int GENERATED BY DEFAULT AS IDENTITY,
"new_city_id" int,
PRIMARY KEY("city_id")
);