What to do when a defect has been discovered in a release version of Besu?

GitHub

Nothing. 

The history is GitHub (release tags and commits) are never rewritten when a defect or bug is discovered. It serves as a record of history, documented what has happened over the course of Besu's lifetime.

Binary Artefact

When a defect is found that causes data corruption in common cases or significant security breach, then that artefact may be removed from the Binary Artefact store.

Changelog and Communication 

For significant defects including where a release is not usable in common cases, the Changelog is updated and a post is made to the besu and besu-contributors channels on RocketChat.