I discovered this showstopper on Friday while working with real data in development of a new app. I managed to narrow it down today and filed the bug report.

https://mariadb.atlassian.net/browse/MDEV-7650

Looks like Maria saves an illegally formatted dyncol string if a dynamic column is longer than 64kB and is not alphabetically last by name among the dynamic column in the blob.

Tbh, even is this were fixed tomorrow so I could proceed with my work, I'm not sure I would use dyncols. My confidence in the feature has been shaken. It's a pity because the Active Record ORM extension I wrote was working.

Tom