Mysql GROUP_CONCAT incomplete, well under group_concat_max_len

Question

Issue is shown query below.

The table has a column value_mean which is a float.

The issue is the last GROUP_CONCAT, it cut off the last 2 characters. Why?

MariaDB [testdb]> SELECT
    a.value_mean,
    b.value_mean,
    HEX(a.value_mean),
    HEX(b.value_mean),
    a.value_mean / b.value_mean,
    a.value_mean / b.value_mean * 100 - 100,
    CONCAT(a.value_mean / b.value_mean, 'AFTER'),
    CONCAT(a.value_mean / b.value_mean * 100 - 100, 'AFTER'),
    GROUP_CONCAT( CONCAT(a.value_mean / b.value_mean, 'AFTER') ),
    GROUP_CONCAT( CONCAT(a.value_mean / b.value_mean * 100 - 100, 'AFTER') )
FROM
    baseline_result AS a
    JOIN baseline_result AS b
WHERE
    a.id = 125716755
    AND b.id=125717382
\G
*************************** 1. row ***************************
                                                            value_mean: 15141600000
                                                            value_mean: 15141600000
                                                     HEX(a.value_mean): 38681F000
                                                     HEX(b.value_mean): 38681F400
                                           a.value_mean / b.value_mean: 0.9999999323715897
                               a.value_mean / b.value_mean * 100 - 100: -0.0000067628410249653825
                          CONCAT(a.value_mean / b.value_mean, 'AFTER'): 0.9999999323715897AFTER
              CONCAT(a.value_mean / b.value_mean * 100 - 100, 'AFTER'): -0.0000067628410249653825AFTER
            GROUP_CONCAT(CONCAT(a.value_mean / b.value_mean, 'AFTER')): 0.9999999323715897AFTER
GROUP_CONCAT(CONCAT(a.value_mean / b.value_mean * 100 - 100, 'AFTER')): -0.0000067628410249653825AFT
1 row in set (0.001 sec)

This shows it even better but harder to follow.. the SELECT will now actually have two rows to GROUP_CONCAT and can see the cutoff characters are in the middle of the GROUP_CONCAT:

MariaDB [testdb]> SELECT
    a.value_mean,
    b.value_mean,
    HEX(a.value_mean),
    HEX(b.value_mean),
    a.value_mean / b.value_mean,
    a.value_mean / b.value_mean * 100 - 100,
    CONCAT(a.value_mean / b.value_mean, 'AFTER'),
    CONCAT(a.value_mean / b.value_mean * 100 - 100, 'AFTER'),
    GROUP_CONCAT(CONCAT(a.value_mean / b.value_mean, 'AFTER')),
    GROUP_CONCAT(CONCAT(a.value_mean / b.value_mean * 100 - 100, 'AFTER'))
FROM
    baseline_result AS a
    JOIN baseline_result AS b
WHERE
    a.id = 125716755
    AND b.id in (125717382, 125717383)
\G
*************************** 1. row ***************************
                                                            value_mean: 15141600000
                                                            value_mean: 15141600000
                                                     HEX(a.value_mean): 38681F000
                                                     HEX(b.value_mean): 38681F400
                                           a.value_mean / b.value_mean: 0.9999999323715897
                               a.value_mean / b.value_mean * 100 - 100: -0.0000067628410249653825
                          CONCAT(a.value_mean / b.value_mean, 'AFTER'): 0.9999999323715897AFTER
              CONCAT(a.value_mean / b.value_mean * 100 - 100, 'AFTER'): -0.0000067628410249653825AFTER
            GROUP_CONCAT(CONCAT(a.value_mean / b.value_mean, 'AFTER')): 0.9999999323715897AFTER,0.8918810968677036AFTER
GROUP_CONCAT(CONCAT(a.value_mean / b.value_mean * 100 - 100, 'AFTER')): -0.0000067628410249653825AFT,-10.81189031322964AFTER
1 row in set (0.001 sec)

[root@test ~]# mysql --version
mysql  Ver 15.1 Distrib 5.5.56-MariaDB, for Linux (x86_64) using readline 5.1

I'm a bit stumped by this. If the float value are slightly different and issue is avoided. Just been getting lucky, I guess. Larger query failed, narrowed issue down to these two floats.. but now bit stumped.

I can avoid issue by rounding the values that are GROUP_CONCAT-ed but that isn't ideal solution.

More info.. the column is float, but just selecting value_mean its close enough it prints the same. Printing HEX() you can see value better.

MariaDB [testdb]> SELECT
id,
value_mean,
HEX(value_mean)
FROM baseline_result
WHERE id in (125716755,125717382,125717383);
+-----------+-------------+-----------------+
| id        | value_mean  | HEX(value_mean) |
+-----------+-------------+-----------------+
| 125716755 | 15141600000 | 38681F000       |
| 125717382 | 15141600000 | 38681F400       |
| 125717383 | 16977100000 | 3F3EA2800       |
+-----------+-------------+-----------------+
3 rows in set (0.000 sec)

MariaDB [testdb]> SELECT VERSION();
+----------------+
| VERSION()      |
+----------------+
| 5.5.56-MariaDB |
+----------------+
1 row in set (0.000 sec)

Here is easier repro with more recent MariaDB:

MariaDB [(none)]> SELECT VERSION();
+---------------------------------------+
| VERSION()                             |
+---------------------------------------+
| 10.5.13-MariaDB-1:10.5.13+maria~focal |
+---------------------------------------+
1 row in set (0.000 sec)

MariaDB [(none)]> select GROUP_CONCAT(a) FROM ( SELECT CONCAT(conv('38681F000', 16, 10) / conv('38681F400', 16, 10) * 100 - 100, 'AFTER') as a) xxx;
+------------------------------+
| GROUP_CONCAT(a)              |
+------------------------------+
| -0.0000067628410249653825AFT |
+------------------------------+
1 row in set (0.000 sec)