CacheManager 'assets' cache is not configurable in ext_localconf.php
The IconRegisty is loaded in backend mode and reads the configuration (caches from 'assets') during object construction.
IconRegistry is usually instantiated during ext_localconf.php (due to extensions registering icons),
and therefore create's the 'assets' cache during ext_localconf.php loading.
After CacheManager has created the assets cache once, it will never be recreated again,
when the final configuration is set, after all ext_localconf.php files have been loaded.