pl/pgsql errors when multi-dimensional arrays are used

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/13/arrays.html
Description:

I have PostgreSQL 13. Let's declare the type below, then use it in
pl/pgsql:

create type typ1 as (
fld1 int[][]
);

do $$
declare
a typ1;
begin
a.fld1 = '{{121,122,123,124}}'; -- OK (1)
a.fld1[1] = '{221,222,223,224}'; -- fails (2)
a.fld1[1][1] = 321; -- OK (3)
a.fld1[1][2] = 322; -- OK unless line (1) is removed
end;
$$;

In line (2) the plql reports ERROR: invalid input syntax for type integer:
"{221,222,223,224}"
When lines (1) and (2) are removed, psql reports ERROR: array subscript out
of range

Is this expected behavior? Why?

Rafal