On a production server, we experience a crash during an alter table. I have attached the relevant entries from syslog (which is where MariaDB logs to), along with a similar SQL creation statement for the table that was altered. Unfortunately, the table in question was marked as crashed after the service restarted, and there was multiple entries per second being written to the error log about the table being crashed. Another member of the development team made the decision to drop the original table, and we don't have a backup of that table (it is a new table, being used as a summary table).
The main known difference between the attached SQL for the table and the original is that the original had Foreign Keys to other tables. If you feel this might be relevant, I can attempt to find the foreign keys that were in place, along with the table definitions of those the target tables.
MDEV-14021 has a similar crash (same assertion fail), so setting as relates to (I don't know enough about this bug to say if it is the same or not).