This seems to be a regression introduced in 21c or 19c. I cannot reproduce it with 18c. Try this sequence of statements:
create table t (i int primary key, j int);
create table u (x int references t, y int);
insert into t (i) values (2);
insert into u (x, y) values (2, 2);
update t
set t.j = (select y from u where u.x = t.i)
where t.i = 2;
insert into u (x, y) values (2, 3);
update t
set t.j = (select y from u where u.x = t.i)
where t.i = 2;
The last update statement produces an update count of 2 with Oracle 21c and 23c (using https://hub.docker.com/r/gvenzl/oracle-free or dbfiddle). It produces ORA-01427 in 18c (see https://dbfiddle.uk/pvqC6bNT).
Running the last update statement again produces the following error on 21c and 23c:
ORA-30926: The operation attempted to update the same row (rowid: 'AAASDOAAPAAAANfAAA') twice.
Which I guess is an OK alternative to ORA-04127