Why does this stored procedure work with one query but not another?, Sql Server

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create function [dbo].[delimitedsplit8K] (
          @pstring varchar(8000)
        , @pdelimiter char(1)
      )
    returns table with schemabinding as
     return
      with e1(N) as (
        select 1 union all select 1 union all select 1 union all 
        select 1 union all select 1 union all select 1 union all 
        select 1 union all select 1 union all select 1 union all select 1
      )
      , e2(N) as (select 1 from e1 a, e1 b)
      , e4(N) as (select 1 from e2 a, e2 b)
      , ctetally(N) as (
        select top (isnull(datalength(@pstring),0)) 
          row_number() over (order by (select null)) from e4
      )
      , ctestart(N1) as (
        select 1 union all
        select t.N+1 from ctetally t where substring(@pstring,t.N,1) = @pdelimiter
      )
      , ctelen(N1,L1) as (
        select s.N1,
          isnull(nullif(charindex(@pdelimiter,@pstring,s.N1),0)-s.N1,8000)
        from ctestart s
      )
     select itemnumber = row_number() over(order by l.N1)
          , item       = substring(@pstring, l.N1, l.L1)
       from ctelen l
    ;
    go
  declare @diesection varchar(8000) = '1,2,3'
  select s.ItemNumber, s.Item
  from [dbo].[delimitedsplit8K](@diesection,',') s
/* ------- in () example ---- */
create table t (column1 int); 
insert into t values (1),(2),(3),(4);

select *
from t
where column1 in (
    select Item 
    from dbo.delimitedsplit8K(@diesection,',')
    )

View schema
Execution time: 0,03 sec, rows selected: 7, rows affected: 4, absolute service time: 0,17 sec