While issuing ALTER on a InnoDB table with FTS MariaDB breaks with a segmentation fault:

kernel: mysqld[812]: segfault at 0 ip 00005649f0b31ac8 sp 00007febffffd0b0 error 6 in mysqld[5649efe3e000+12c9000]

MariaDB [test]> optimize table t;
-----------------------------------------------------------------------------------------+

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

-----------------------------------------------------------------------------------------+
2 rows in set (10.312 sec)

MariaDB [test]> optimize table t;
-----------------------------------------------------------------------------------------+

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

-----------------------------------------------------------------------------------------+
2 rows in set (6.379 sec)

MariaDB [test]> optimize table t;
ERROR 2013 (HY000): Lost connection to MySQL server during query
MariaDB [test]> optimize table t;
ERROR 2013 (HY000): Lost connection to MySQL server during query
MariaDB [test]> optimize table t;
ERROR 2013 (HY000): Lost connection to MySQL server during query

We managed to replicate this after around 3 consecutive calls of ALTER TABLE t ENGINE=InnoDB; after the initial data import.

If we remove the FTS index there are no more issues.
It seems the issue appears on FTS index merge, recreate.

I've attached to this ticket the backtrace, MariaDB log, configuration and sample data used to replicate the issue.

If you require any additional info, just let me know.