Consider the following simple scenario:

PREPARATIONS:
1) Let's say we have a table USERS with one COLUMN NAME: create table USERS(NAME varchar(100));
2) Let's put some values there:

insert into USERS(name) values('User1');
insert into USERS(name) values('User2');
insert into USERS(name) values('User3');
insert into USERS(name) values('User4');
insert into USERS(name) values('User5');
insert into USERS(name) values('User6');
insert into USERS(name) values('User7');
insert into USERS(name) values('User8');
insert into USERS(name) values('User9');
insert into USERS(name) values('User10');
insert into USERS(name) values('User11');
insert into USERS(name) values('User12');
...
up to 10 000 000 values...

PROBLEM:
I am using old but kinda "standard" way of paging WITHOUT OVER, here example of paging of (0, 5> users:
select * from (select NAME, ROWNUM r from users where rownum <= 5) where r > 0;

Some users are returned, let's say:

NAME R
User1 1
User2 2
User3 3
User4 4
User5 5

But the "problem?" happens when I change the boundaries, let's say to (0, 8> users:
select * from (select NAME, ROWNUM r from users where rownum <= 8) where r > 0;
NAME R
User3 1
User4 2
User5 3
User6 4
User7 5
User8 6
User9 7
User10 8

As you can see, DIFFERENT users are returned, for example User1 is not returned, even it was returned in the first query.
The problem IS NOT replicable with such simple data set, but I replicated with set of 13 millions of rows.
Results returned for (0, 2 000 000> are not contained in set returned for (0, 10 000 000)


QUESTION:
As you can see the query HAS NO ORDERING so based on the definition the result can be in ANY ordering, rownum just cuts result when predicates applies - so from my understanding it is not bug, but need confirmation from you, experts. Is it valid to assume that Oracle optimiser just decides to use different way of execution for boundaries (0, 2 000 000> compare to a case when boundaries are (0, 10 000 000>? If so, is the only way how to fix it use of ROW_NUMBER OVER some ordering or alternatively order by BEFORE any rownum?

Thanks a lot for your answer.

NOTES:
I am using Oracle 12.1.0.2. Table has no primary keys. The problem is ALWAYS replicable in one environment and never replicable in another environments with the same data, Oracle version and patches...

SQL> create table t as
  2  select mod(rownum,2) col, d.* from dba_objects d;

Table created.

SQL>
SQL> create index ix on t ( col );

Index created.

SQL>
SQL> set autotrace traceonly explain
SQL> select * from t where col = 0 and rownum <= 1;

Execution Plan
----------------------------------------------------------
Plan hash value: 658510075

---------------------------------------------------------------------------------------------
| Id  | Operation                            | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                     |      |     1 |   135 |     2   (0)| 00:00:01 |
|*  1 |  COUNT STOPKEY                       |      |       |       |            |          |
|   2 |   TABLE ACCESS BY INDEX ROWID BATCHED| T    |     1 |   135 |     2   (0)| 00:00:01 |
|*  3 |    INDEX RANGE SCAN                  | IX   |       |       |     1   (0)| 00:00:01 |
---------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter(ROWNUM<=1)
   3 - access("COL"=0)

SQL> select * from t where col = 0 and rownum <= 100000;

Execution Plan
----------------------------------------------------------
Plan hash value: 508354683

---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      | 39201 |  5168K|   432   (1)| 00:00:01 |
|*  1 |  COUNT STOPKEY     |      |       |       |            |          |
|*  2 |   TABLE ACCESS FULL| T    | 39201 |  5168K|   432   (1)| 00:00:01 |
---------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - filter(ROWNUM<=100000)
   2 - filter("COL"=0)