BUG #14059: BUG? function in select clause called more times when use offset

The following bug has been logged on the website:

Bug reference: 14059
Logged by: digoal
Email address: digoal(at)126(dot)com
PostgreSQL version: 9.5.2
Operating system: CentOS 6.x x64
Description:

postgres=# create or replace function f() returns void as $$
declare
begin
raise notice 'called';
end;
$$ language plpgsql strict volatile;
CREATE FUNCTION

postgres=# select f(),* from ( values(1),(2),(3),(4),(5),(6) ) t(id) offset
3 limit 2;
NOTICE: called
NOTICE: called
NOTICE: called
NOTICE: called
NOTICE: called
f | id
---+----
| 4
| 5
(2 rows)

offset skip 3 tuple, but function f() called with 5 times, can we tuning
this .
or this is a bug?