MySQL using a multi-column index even when the first column isn't being queried

I have MySQL version 8.0.37. From what I understand about a multi-column index in this version, it will be used by MySQL ONLY if the query contains a subset of all the columns, starting from the first. For ex, I have this index in my InnoDB table

mysql> show indexes from my_table;
+------------------------+------------+------------------------------------------------------------+--------------+-----------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| Table    | Non_unique | Key_name                        | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
+------------------------+------------+------------------------------------------------------------+--------------+-----------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| my_table |          0 | PRIMARY                         |            1 | id          | A         |       32643 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| my_table |          1 | my_table_entity_id              |            1 | entity_id   | A         |       20160 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| my_table |          1 | my_table_entity_id_sub_id_value |            1 | entity_id   | A         |       18222 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| my_table |          1 | my_table_entity_id_sub_id_value |            2 | sub_id      | A         |       32985 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
| my_table |          1 | my_table_entity_id_sub_id_value |            3 | value       | A         |       32545 |     NULL |   NULL |      | BTREE      |         |               | YES     | NULL       |
+------------------------+------------+------------------------------------------------------------+--------------+-----------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+

With my_table_entity_id_sub_id_value index, I can run queries over entity_id, or both entity_id and sub_id or all the 3 columns. This is also what the MySQL [documentation](https://dev.mysql.com/doc/refman/8.4/en/multiple-column-indexes.html) says. However, this is the explain analyze output of a query over only the 2nd and 3rd columns, i.e., sub_id and value, and still the index is being used.

mysql> explain analyze select distinct entity_id from my_table where sub_id = 107 and value  = 'd90e7a26-2fc5-4e16-87c5-a2e9da5a26f7';
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| EXPLAIN                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| -> Group (no aggregates)  (cost=3552 rows=330) (actual time=3.52..14.7 rows=3103 loops=1)
    -> Filter: ((my_table.value = 'd90e7a26-2fc5-4e16-87c5-a2e9da5a26f7') and (my_table.sub_id = 107))  (cost=3519 rows=330) (actual time=3.44..14.3 rows=3103 loops=1)
        -> Covering index scan on my_table using my_table_entity_id_sub_id_value  (cost=3519 rows=32985) (actual time=0.0741..10.4 rows=33202 loops=1)
 |
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.39 sec)

I realise it's a "covering index" scan. What I understand about them is that they're used for retrieving values directly from the index, so I understand entity_id which I'm selecting is in that index. However, the where is still over only the 2nd and 3rd columns, and that is the filtration criteria. Am I missing something here? What am I not understanding about covering index scans?