Postgres partitioned table query scans all partitions instead of one - postgresql

Question

I have a table
CREATE TABLE IF NOT EXISTS prices
(shop_id integer not null,
good_id varchar(24) not null,
eff_date timestamp with time zone not null,
price_wholesale numeric(20,2) not null default 0 constraint chk_price_ws check (price_wholesale >= 0),
price_retail numeric(20,2) not null default 0 constraint chk_price_rtl check (price_retail >= 0),
constraint pk_prices primary key (shop_id, good_id, eff_date)
)partition by list (shop_id);
CREATE TABLE IF NOT EXISTS prices_1 partition of prices for values in (1);
CREATE TABLE IF NOT EXISTS prices_3 partition of prices for values in (2);
CREATE TABLE IF NOT EXISTS prices_4 partition of prices for values in (3);
CREATE TABLE IF NOT EXISTS prices_4 partition of prices for values in (4);
...
CREATE TABLE IF NOT EXISTS prices_6 partition of prices for values in (100);
I'd like to delete outdated prices. The table is huge , so I try to delete small portions of records.
If I use loop and the variable v_shop_id then after 6 times Postgres starts scanning all partitions. I simplified the code, the real code has inner loop by shop_id.
If I use loop without the variable (I explicitly specify the value) Postgres doesn't scan all partitions
here code with the variable
do $$
declare
v_shop_id integer;
v_date_time timestamp with time zone := now();
begin
v_shop_id := 8;
for step in 1..10 loop
delete from prices p
using (select pd.good_id, max(pd.eff_date) as mxef_dt
from prices pd
where pd.eff_date < v_date_time - interval '30 days'
and pd.shop_id = v_shop_id
group by ppd.good_id
having count(1)>1
limit 40000) pfd
where p.eff_date <= pfd.mxef_dt
and p.shop_id = v_shop_id
and p.good_id = pfd.good_id;
end loop;
end;$$LANGUAGE plpgsql
How can I force Postrges to scan one desired partition only?