Selected timeline reconstructed from the supplied September 9 logs.
This is not the complete debug.log or MariaDB error log.

Connection ID: 888975

10:48, 10:50, 10:56, and 11:19
Multiple INSERT ... SELECT ... ORDER BY statements reported MCS2015 sort
memory limit errors. The normal dbcon End SQL statement was logged for these
errors.

10:57 - 11:17
An INSERT completed successfully after earlier MCS2015 errors. This indicates
that an MCS2015 error alone does not always leave the connection in the state
that causes the crash.

12:06:58
The affected INSERT statement started on connection ID 888975. No normal End
SQL statement was observed before the next start.

12:07:16
The INSERT statement was started again using the same connection ID.

12:07:27
cpimport started.

12:08:09
MariaDB received SIGSEGV / signal 11.

Crash signature:
ha_mcs_impl_write_batch_row_(... cal_connection_info ...)+0x163

The function and offset matched the September 8 incident.