There's a warning on
https://mariadb.com/kb/en/mariadb-1064-release-notes/ (thanks @jimh_@twitter.com) about file corruption on FreeBSD (
https://jira.mariadb.org/browse/MDEV-26537) which might be the same issue. I'll have to debug after my hair is no longer on fire.

Just a heads up for anyone else having MariaDB crashes on OpenBSD 7.0.