Inconsistent data results from ext_mgr->all_available() vs ext_mgr->is_available()

If an extension folder exists, but the data in composer.json is inconsistent, the extension is not listed by the extension manager all_available method, as it checks for that. However, if you single out an extension and check it via is_available, it will return true even if the extension composer.json is malformed.
The same behaviour should be expected from both calls.