select (fn()).* executes function multiple times

PostgreSQL 10.5, 11.1:

create table fn_ret_test_storage (f1 int default 1, f2 int default 2, f3
int default 3);

create or replace function fn_ret_test(out f1 int, out f2 int, out f3 int)
returns record
language plpgsql
as
$$
begin
insert into fn_ret_test_storage default values
returning * into f1, f2, f3;
end
$$;

do
$$
declare
_count int;
begin
select count(*) into _count from fn_ret_test_storage;
perform (fn_ret_test()).*;
select count(*) - _count into _count from fn_ret_test_storage;
raise notice '----------------- % rows inserted -----------------', _count;
end
$$;

-- output: NOTICE: ----------------- 3 rows inserted -----------------