"Sortlen" reports "4" for tis620_thai_ci in this query:

mysql> show collation like 'tis620_thai_ci';

+----------------+---------+----+---------+----------+---------+

| Collation      | Charset | Id | Default | Compiled | Sortlen |

+----------------+---------+----+---------+----------+---------+

| tis620_thai_ci | tis620  | 18 | Yes     | Yes      |       4 |

+----------------+---------+----+---------+----------+---------+

1 row in set (0.00 sec)

"Sortlen" comes from the "strxfrm_multiply" member
of my_charset_tis620_thai_ci.

The code in strings/ctype-tis620.c does not really look like
the weight string for tis620_thai_ci can grow four times comparing
to the original text string. strxfrm_multiply=1 should be enough.

A too large strxfrm_multiply (Sortlen) value affects performance
of non-indexed ORDER BY negatively.