TYPO3 Core

#88366 introduced a new naming for caches and a fallback to handle old names transparently.
#88512 added another piece of code for this old-name schema handling

We have the situtation that an extension is changing the backend

An extension is changing the cache configuration of built-in caches in its ext_localconfig.php like this

$GLOBALS['TYPO3_CONF_VARS']['SYS']['caching']['cacheConfigurations']['cache_pages']['backend'] = \TYPO3\CMS\Core\Cache\Backend\RedisBackend::class;

According to documentation this should still work, but it does not.

While working with the site does cause any errors or log entries the flushing of the caches "does not happen" when editing content.
Example: Updating any content element on a page, does not flush the page's cache entry.

It turns out that the configuration of the caches is faulty, as the setting written above overwrites any existing setting of the cache, like the group.
This causes the experienced behaviour as DataHandler flushes caches by group.

The solution therefore must be to use old cache configurations (for caches starting with cache_*) as an additive override for existing cache configurations.
This way the basic settings from core are preserved.