postgres split and convert string values to composite arrays

It is because you are trying to assign an array value to the record. Try:

do $$
declare
    attr_array t_attr[];
    txt text := 'id1#data1';
begin
    attr_array[1] := (split_part(txt, '#', 1), split_part(txt, '#', 2));
    raise info '%', attr_array;
end $$;

Output:

INFO:  {"(id1,data1)"}
DO

However if you really need to split values using arrays:

do $$
declare
    a text[];
begin
    ....
    a := regexp_split_to_array(txt, '#');
    attr_array[1] := (a[1], a[2]);
end $$;