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 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.