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?

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.