plpgsql plan caching allowing invalid data to enter table?

It's looking like I can use a plpgsql function to insert data into a table
that violates a domain constraint. Is this a known problem?

Session 1:

create domain my_domain text check (length(value) > 2);
create table my_table (name my_domain);

create function f(text) returns void as $$
declare my_var my_domain := $1;
begin
insert into my_table values (my_var);
end $$ language plpgsql;

Session 2:
select f('test');
delete from my_table;
-- Keep session open!

Session 1:
alter domain my_domain drop constraint my_domain_check;
alter domain my_domain add constraint my_domain_check check (length(value)
> 5);

Session 2:
select f('test');
-- This works, but it should fail.
-- I have a constraint of more than 5 characters on the domain.
-- But I can insert a row with 4 characters.