dolt_patch() attempts to re-create an index

I have two versions of a table schema, old and new, in branches of those names. In old, the description for a table is:

```OLD

+------------------------+--------------------------------------------------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------------------+--------------------------------------------------------------+------+-----+---------+----------------+
| field_name | int unsigned | NO | UNI | NULL | |
+------------------------+--------------------------------------------------------------+------+-----+---------+----------------+
INDEXES
+------------+------------+--------------------------+--------------+------------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
+------------+------------+--------------------------+--------------+------------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| table_name | 0 | field_name_UNIQUE | 1 | field_name | NULL | 0 | NULL | NULL | | BTREE | | | YES | NULL |
+------------+------------+--------------------------+--------------+------------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
```

And in the new version,

```NEW
+------------------------+--------------------------------------------------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------------------+--------------------------------------------------------------+------+-----+---------+----------------+
| field_name | int unsigned | NO | UNI | NULL | |
+------------------------+--------------------------------------------------------------+------+-----+---------+----------------+
INDEXES
+------------+------------+--------------------------+--------------+------------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression |
+------------+------------+--------------------------+--------------+------------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
| table_name | 0 | field_name_UNIQUE | 1 | field_name | NULL | 0 | NULL | NULL | | BTREE | | | YES | NULL |
+------------+------------+--------------------------+--------------+------------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+
```
So why, when I call:
``` dolt sql -q "SELECT statement FROM dolt_patch('old', 'new', 'table_name') WHERE diff_type = 'schema';" --result-format=csv```

Do I get this:

```ALTER TABLE `table_name` DROP INDEX `field_name_UNIQUE`;```
```ALTER TABLE `table_name` ADD INDEX `field_name_UNIQUE`(`field_name`);```

Why is there any need to destroy and recreate the index?

What's more, if I do NOT specify the table name, and just call:
``` dolt sql -q "SELECT statement FROM dolt_patch('old', 'new') WHERE diff_type = 'schema';" --result-format=csv```
The `DROP` disappears, and we are left only with the `ADD INDEX`, which tries to create an index which is already there.