Test case:

create table t1 (col1 int);
create index i1 on t1 (col1);
commit;
insert into t1 (col1) values (1);
commit;
create table t2 (col2 int);
commit;

select case when exists (select 1 from t1 where col1 = col2) then 1 else 0 end from t2;
-- PLAN (T1 INDEX (I1))
-- PLAN (T2 NATURAL)
-- Everything is okay, T1 is accessed via an index

insert into t2 (col2) values (1) returning case when exists (select 1 from t1 where col1 = col2) then 1 else 0 end;
-- PLAN (T1 NATURAL)
-- Full scan is chosen, this is a bug