Why does the MySQL (MyISAM) index break, and how to avoid it?

Question

MySQL

I noticed that one large composite index in MySQL (MyISAM table) does not work for me. If you display SHOW INDEX, you can see that the number of Cardinality (number of unique values) on all index fields except one is reset to NULL. I did an OPTIMIZE TABLE, the index was fixed, but broke again within a day.
Can anyone suggest what is causing this and how to avoid it?
PS There is no way to switch to InnoDB.

Answer 1

dev.mysql.com/doc/refman/5.7/en/myisam-table-probl...
usually with large tables, and even write-intensive ones