Postgres query won't finish in function but if run separately it works

This is simplified version of my function with the query in it (therefore any variables are now useless) and this function won't finished but if I run the same query separately it finishes in under a second.

function that never finishes

select * from test_function_difference(1);


CREATE OR REPLACE FUNCTION test_function_difference ( 
  p_does_nothing int

)  
RETURNS TABLE(
  t_datum date,
  t_capacity numeric,
  t_used numeric,
  t_category int,
  t_category_name text,  
  t_used_p numeric,
  t_unused_p numeric
)
  VOLATILE
AS $dbvis$

declare
p_sql text := '';
p_execute text := '';
rec record;
begin

p_sql := 
'
 with 
vytizeni as (
  select 
    date_trunc(''day'',mcz.datum)::date as datum ,  
    sum(zd.v_vytizeni)/3600.0 used
  from v_ui_cdc_s5_misto_cas_zdroj_aggregace mcz
  left join (select * , pul_den as den_noc from v_ui_cdc_s5_misto_cas_zdroj_aggregace_zdrobneni) zd on mcz.id = zd.id
  where
    datum between  ''2018-12-31'' and ''2018-12-31''

    and ( zahranicni = 0 or zahranicni is null )
     and den_noc = -1 
  group by 
    date_trunc(''day'',mcz.datum)::date
)
,kapacita as (
  select
    date_trunc(''day'',datum)::date as datum , 
    sum(obsazeni_g)/3600.0 capacity
  from v_ui_cdc_s5_misto_cas_zdroj_aggregace
  where
    datum between  ''2018-12-31'' and ''2018-12-31''

  group by
    date_trunc(''day'',datum)::date 
)
,zdroj as (
  select 
    k.datum,  
    k.capacity,
    v.used,
    -1 category
  from kapacita k 
  join vytizeni v on k.datum = v.datum
)

select
  c.* , 
  kc.nazev::text categeroy_name,
  case when sum(capacity)over(partition by datum) = 0 then 1 else used/sum(capacity)over(partition by datum) end as used_p,
  greatest(1 - case when sum(capacity)over(partition by datum) = 0 then 1 else sum(used)over(partition by datum)/sum(capacity)over(partition by datum) end,0) as unused_p
from  zdroj  c
left join v_ui_cdc_s5_kategorie_cinnosti kc on kc.id = c.category
order by c.datum
';

raise notice '% ' , p_sql;

RETURN QUERY 
execute p_sql;

END;
$dbvis$ LANGUAGE plpgsql

and the query I run separately (finished in 533 ms)

with 
vytizeni as (
  select 
    date_trunc('day',mcz.datum)::date as datum ,  
    sum(zd.v_vytizeni)/3600.0 used
  from v_ui_cdc_s5_misto_cas_zdroj_aggregace mcz
  left join (select * , pul_den as den_noc from v_ui_cdc_s5_misto_cas_zdroj_aggregace_zdrobneni) zd on mcz.id = zd.id
  where
    datum between  '2018-12-31' and '2018-12-31'

    and ( zahranicni = 0 or zahranicni is null )
     and den_noc = -1 
  group by 
    date_trunc('day',mcz.datum)::date
)
,kapacita as (
  select
    date_trunc('day',datum)::date as datum , 
    sum(obsazeni_g)/3600.0 capacity
  from v_ui_cdc_s5_misto_cas_zdroj_aggregace
  where
    datum between  '2018-12-31' and '2018-12-31'

  group by
    date_trunc('day',datum)::date 
)
,zdroj as (
  select 
    k.datum,  
    k.capacity,
    v.used,
    -1 category
  from kapacita k 
  join vytizeni v on k.datum = v.datum
)

select
  c.* , 
  kc.nazev::text categeroy_name,
  case when sum(capacity)over(partition by datum) = 0 then 1 else used/sum(capacity)over(partition by datum) end as used_p,
  greatest(1 - case when sum(capacity)over(partition by datum) = 0 then 1 else sum(used)over(partition by datum)/sum(capacity)over(partition by datum) end,0) as unused_p
from  zdroj  c
left join v_ui_cdc_s5_kategorie_cinnosti kc on kc.id = c.category
order by c.datum

Edit: I was able to get results from the function after almost 28 minutes (also I tried it on Sunday night, means I had the resources of the whole server, because during normal load function have not finished even after one hour) and right after that I ran query standalone and get results after 2.1 sec Here are the explain analyze

function: 28 minutes https://explain.depesz.com/s/v9xJ

standalone query: 2.1 sec https://explain.depesz.com/s/aBri

second run stand alone 430ms https://explain.depesz.com/s/ENva

Interesting note: if I edit start date for the interval to '2018-12-30' or any other date the function finishes as well

which means

start date = '2018-12-31'
query => finishes under 1 second
function => won't finish
start date = '2018-12-30'
query => finishes under 1 second
function => finishes under 1 second

Version details: PostgreSQL 10.7 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-36), 64-bit