MySQL query using limit and offset does not return expected ordered results

I am trying to get the count of entries by users grouped by year, month and user name from a table which has 45M entries. The query result has around 4M records which I wasn't able to get in one go so I decided to use limit and offset.

To retrieve the first 1M records I've written the query below:

select SQL_BIG_RESULT uis.nick, uis.user_id, CONCAT(t.year, '-', LPAD(t.month, 2, 0)) AS DATE, t.count 
from (select SQL_BIG_RESULT e.user_id, YEAR(e.created_at) as year, MONTH(e.created_at) as month, COUNT(*) AS count
        from entries e
        group by YEAR(e.created_at), MONTH(e.created_at), e.user_id
        limit 1000000
     ) t
inner join users u on u.id = t.user_id
inner join user_infos ui on ui.user_id = u.id
inner join user_identifiers uis on uis.user_info_id = ui.id
order by t.year, t.month, uis.nick;

To retrieve the second 1M records I've set an offset of 999998 so I would have 2 overlapping rows so that I could double check that it's correct, hence this query below:

select SQL_BIG_RESULT uis.nick, uis.user_id, CONCAT(t.year, '-', LPAD(t.month, 2, 0)) AS DATE, t.count 
from (select SQL_BIG_RESULT e.user_id, YEAR(e.created_at) as year, MONTH(e.created_at) as month, COUNT(*) AS count
        from entries e
        group by YEAR(e.created_at), MONTH(e.created_at), e.user_id
        limit 999998, 1000000
     ) t
inner join users u on u.id = t.user_id
inner join user_infos ui on ui.user_id = u.id
inner join user_identifiers uis on uis.user_info_id = ui.id
order by t.year, t.month, uis.nick;

Then to compare the results and double check, I've got the tail of the first 1M records and the head of the second 1M records. There should be 2 overlapping records in my understanding -since I've used an offset of 999998- but there is something wrong.

It's also evident that there is something wrong with the query because the first file ends with zzzzz but then the second file starts with 0 3 kalem ucu which should not be after z in alphabetical order.

$ tail entry_counts_by_users_1_1m.csv

| user_nick   | user_id | date    | entry_count | 
|-------------|---------|---------|-------------| 
| zskal       | 493395  | 2013-05 | 8           | 
| zuhanzee    | 397659  | 2013-05 | 2           | 
| zulmet      | 446672  | 2013-05 | 74          | 
| zuluuuuuu   | 1240043 | 2013-05 | 9           | 
| zverkov     | 502616  | 2013-05 | 2           | 
| zvezdite    | 750458  | 2013-05 | 1           | 
| zx          | 249598  | 2013-05 | 15          | 
| zyprexa 5mg | 779519  | 2013-05 | 16          | 
| zzgx        | 584985  | 2013-05 | 2           | 
| zzzzz       | 22730   | 2013-05 | 1           | 

$ head entry_counts_by_users_1m_2m.csv

| nick          | user_id | DATE    | count | 
|---------------|---------|---------|-------| 
| 0 3 kalem ucu | 624699  | 2013-05 | 4     | 
| 0132          | 995914  | 2013-05 | 3     | 
| 03072010      | 960606  | 2013-05 | 9     | 
| 0312020008    | 804486  | 2013-05 | 2     | 
| 0326          | 446816  | 2013-05 | 1     | 
| 05            | 575534  | 2013-05 | 1     | 
| 05012009      | 1171153 | 2013-05 | 6     | 
| 0904          | 514964  | 2013-05 | 2     | 
| 0kmzeka       | 777191  | 2013-05 | 4     | 

Could you help me understand what I am doing wrong here?

+-----------+
| @@version |
+-----------+
| 8.0.19    |
+-----------+

UPDATE

These are the results I get after using ORDER BY in my subquery:

select SQL_BIG_RESULT uis.nick, uis.user_id, CONCAT(t.year, '-', LPAD(t.month, 2, 0)) AS DATE, t.count
    from (select SQL_BIG_RESULT e.user_id, YEAR(e.created_at) as year, MONTH(e.created_at) as month, COUNT(*) AS count
            from entries e
            group by YEAR(e.created_at), MONTH(e.created_at), e.user_id
            order by year, month, user_id
            limit 1000000) t
    inner join users u on u.id = t.user_id
    inner join user_infos ui on ui.user_id = u.id
    inner join user_identifiers uis on uis.user_info_id = ui.id

For the first 1M records:

$ tail entry_counts_by_users_1_1m.csv

| user_name                  | user_id | date    | entry_count | 
|----------------------------|---------|---------|-------------| 
| statistic er               | 667546  | 2012-06 | 1           | 
| mula                       | 612905  | 2013-02 | 1           | 
| sisman cirkin bi de kezban | 1327434 | 2013-02 | 2           | 
| tyra34                     | 1329280 | 2013-03 | 1           | 
| ecemazkan                  | 1332628 | 2013-02 | 1           | 
| susamlicubuk               | 1333079 | 2013-02 | 1           | 
| hemenhemenherterim         | 631784  | 2011-04 | 1           | 
| umursamaz tavrin hastasi   | 1060158 | 2012-09 | 2           | 
| uslucocuk                  | 1254758 | 2012-09 | 1           | 
| dharamsala                 | 956110  | 2012-09 | 1           | 

select SQL_BIG_RESULT uis.nick, uis.user_id, CONCAT(t.year, '-', LPAD(t.month, 2, 0)) AS DATE, t.count
    from (select SQL_BIG_RESULT e.user_id, YEAR(e.created_at) as year, MONTH(e.created_at) as month, COUNT(*) AS count
            from entries e
            group by YEAR(e.created_at), MONTH(e.created_at), e.user_id
            order by year, month, user_id
            limit 999998, 1000000) t
    inner join users u on u.id = t.user_id
    inner join user_infos ui on ui.user_id = u.id
    inner join user_identifiers uis on uis.user_info_id = ui.id

For the second 1M records:

$ head entry_counts_by_users_1m_2m.csv

| user_name | user_id | date    | entry_count | 
|-----------|---------|---------|-------------| 
| ssg       | 8097    | 2013-06 | 101         | 
| ssg       | 8097    | 2013-07 | 73          | 
| ssg       | 8097    | 2013-08 | 100         | 
| ssg       | 8097    | 2013-09 | 88          | 
| ssg       | 8097    | 2013-10 | 84          | 
| ssg       | 8097    | 2013-11 | 54          | 
| ssg       | 8097    | 2013-12 | 64          | 
| ssg       | 8097    | 2014-01 | 78          | 
| ssg       | 8097    | 2014-02 | 31          | 

I still don't get what I am doing wrong.