`DISTINCT` after `GROUP BY` incorrectly ignores a case-sensitive collation

MariaDB can incorrectly remove rows when a query combines `DISTINCT` with a
`GROUP BY` that contains both a computed expression and the expression's raw
source column. In this form, duplicate elimination treats a case-sensitive
string grouping column as case-insensitive.

The table column below uses `utf8mb4_0900_as_cs`, so `sample_a` and `sample_A`
are distinct values. The grouped relation contains four rows (two labels and
two square-root values). MariaDB's direct `DISTINCT` query returns only two
rows, retaining one case variant for each square-root value. Moving the same
grouped query behind a derived table or temporary table returns all four rows.

This is a MariaDB execution bug. The SQL is valid, and the materialized form
does not introduce extra groups; it avoids the incorrect direct `DISTINCT`
deduplication path.

How to repeat

```sql

DROP DATABASE IF EXISTS mariadb_distinct_group_test;

CREATE DATABASE mariadb_distinct_group_test

    CHARACTER SET utf8mb4

    COLLATE utf8mb4_0900_as_cs;

USE mariadb_distinct_group_test;

CREATE TABLE labels (

    c6 VARCHAR(10)

       CHARACTER SET utf8mb4

       COLLATE utf8mb4_0900_as_cs NOT NULL

);

CREATE TABLE vals (

    c3 DECIMAL(10,2) NOT NULL

);

INSERT INTO labels (c6) VALUES ('sample_a'), ('sample_A');

INSERT INTO vals (c3) VALUES (1.00), (4.00);

```

First execute the grouped query without `DISTINCT`:

```sql

SELECT l.c6, SQRT(v.c3) AS s

FROM labels AS l

CROSS JOIN vals AS v

GROUP BY l.c6, SQRT(v.c3), v.c3;

```

It returns four rows:

```text

sample_a  1

sample_a  2

sample_A  1

sample_A  2

```

Now execute the equivalent direct `DISTINCT` query:

```sql

SELECT DISTINCT l.c6, SQRT(v.c3) AS s

FROM labels AS l

CROSS JOIN vals AS v

GROUP BY l.c6, SQRT(v.c3), v.c3;

```

On MariaDB 12.3.2 this returns only two rows, for example:

```text

sample_a  1

sample_a  2

```

The derived-table and temporary-table forms preserve all four rows:

```sql

SELECT DISTINCT q.c6, q.s

FROM (

    SELECT l.c6, SQRT(v.c3) AS s

    FROM labels AS l

    CROSS JOIN vals AS v

    GROUP BY l.c6, SQRT(v.c3), v.c3

) AS q;

CREATE TEMPORARY TABLE grouped_cut AS

SELECT l.c6, SQRT(v.c3) AS s, v.c3 AS group_source

FROM labels AS l

CROSS JOIN vals AS v

GROUP BY l.c6, SQRT(v.c3), v.c3;

SELECT DISTINCT c6, s

FROM grouped_cut;

```

As an additional control, forcing a binary comparison also returns four rows:

```sql

SELECT DISTINCT BINARY l.c6, SQRT(v.c3) AS s

FROM labels AS l

CROSS JOIN vals AS v

GROUP BY l.c6, SQRT(v.c3), v.c3;

```

1. Expected result

All equivalent forms should return four rows. The case-sensitive collation
requires `sample_a` and `sample_A` to remain distinct, and the raw `v.c3`
grouping key must not change the projected `DISTINCT` result.

1. Actual result

MariaDB 12.3.2 returns only two rows for the direct `DISTINCT` query, merging
the two case variants. The derived-table and temporary-table forms return four
rows.