TYPO3 Core

When editing a page and changing its slug, then saving the page record, then a new redirect is added by \TYPO3\CMS\Redirects\Service\SlugService::createRedirect.

However, the redirects cache is not cleared (\TYPO3\CMS\Redirects\Service\RedirectCacheService::rebuild is not invoked), and the page whose slug was changed (requested via its old URI) consistently throws a HTTP 404 Not Found instead of redirecting to its new URI.

Until:

• Either the entire 'pages' cache is cleared, in which the redirects are serialized under the 'redirects' key;
• Or until a sys_redirects record is modified and saved, which invokes \TYPO3\CMS\Redirects\Hooks\DataHandlerCacheFlushingHook::rebuildRedirectCacheIfNecessary.

Saving a page record with modified slug does not invoke \TYPO3\CMS\Redirects\Hooks\DataHandlerCacheFlushingHook::rebuildRedirectCacheIfNecessary, because \TYPO3\CMS\Redirects\Service\SlugService::createRedirect does not use the DataHandler to store the newly added sys_redirect record:
DataHandlerCacheFlushingHook only listens to DataHandler commands involving sys_redirect records being saved. This is not the case here.
Instead, \TYPO3\CMS\Redirects\Service\SlugService::createRedirect inserts the new record via the \TYPO3\CMS\Core\Database\ConnectionPool API.

Solution:

Call \TYPO3\CMS\Redirects\Service\RedirectCacheService::rebuild in \TYPO3\CMS\Redirects\Service\SlugService::rebuildSlugsForSlugChange just before $this->sendNotification();.