Postgres 12 RLS

Hi,

I'm having a little trouble with RLS in Postgres 12, although first time I've used RLS, so it might just be me !

The problem is that I am calling a function from a web-app, but the function seems to be executing as "postgres" even
thouhgthe web-app logs in as a completely different role ? 

This means that current_user in the function resolves to "postgres" instead of the app user.

This is an example of a function :
create function addses(p_regid text,p_msgid text,p_reqid text) returns integer AS $$
BEGIN
UPDATE foo_regs set reg_aws_ses_msgid=p_msgid,reg_aws_amzn_requestid=p_reqid where uuid=p_regid;
        IF FOUND THEN
                return 1;
        ELSE
                return 0;
        END IF;
END;
$$ LANGUAGE plpgsql SECURITY DEFINER;
grant execute on function addses(p_regid text,p_msgid text,p_reqid text) to myappuser;

The foo_regs table has the following RLS policy:
Policies:
    POLICY "foo_regs_policy"
      USING (((event_id = CURRENT_USER) AND (reg_hide IS FALSE)))