When an audit has been created using chrome.experimental.devtools.audits.addCategory(), if devtools starts with the 'Audits' tab open, it is necessary to navigate away from the tab and back to get the audit to show up. Similarly with disabling the extension; the extension audit remains visible until the user navigates away from the tab and back again.
Closing as invalid, as this bug pertains to the old inspector UI and/or its tests. Please file a new bug (https://www.webkit.org/new-inspector-bug) if the bug/feature/issue is still relevant to WebKit trunk.