create or alter function dbo.wtf(@i int, @str nvarchar(4000), @sep char(1))
returns table as 
return (select i = @i, v = [value] from string_split(@str, @sep))
go

declare @i int = 1
declare @t table(s nvarchar(4000))

insert into @t(s) values (null)

select * from @t t outer apply dbo.wtf(@i, t.s, '|') -- WTF???
select * from @t t outer apply dbo.wtf(1, t.s, '|') -- Ожидаемо

go
drop function dbo.wtf