recreate table t (id int, sss varchar(255))
create unique descending index t_id_desc on t (id)
create unique ascending index t_id_asc on t (id)
create unique descending index t_id_sss_desc on t (id, sss)
create unique ascending index t_id_sss_asc on t (id, sss)
commit

execute block as
declare n int = 0;
begin
  while (n < 10000) do
  begin
    insert into t values (:n, :n);
    n = n + 1;
  end

  n = 0;
  while (n < 10000) do
  begin
    insert into t values (null, null);
    n = n + 1;
  end
end
commit

execute block as
declare n int = 5000;
begin
  while (n > 0) do
  begin
    n = n - 1;
    update t set id = null, sss = null where id = :n;
  end
end
commit


Validation reported

Summary of validation errors
Number of index page errors : 61