I have deleted the repo and re-downloaded to re-do from scratch the issue has disappear

Just udated my Joomla! 5.1 repo and done a composer update the issue is back

[03-Mar-2024 10:21:56 UTC] PHP Fatal error: Access level to Joomla\Plugin\System\Debug\AbstractDataCollector::$defaultDataFormatter must be public (as in class DebugBar\DataCollector\DataCollector) in C:\Users\flo\git\joomla-cms-5.1-dev\plugins\system\debug\src\AbstractDataCollector.php on line 26 [03-Mar-2024 10:21:56 UTC] PHP Stack trace: [03-Mar-2024 10:21:56 UTC] PHP 1. {main}() C:\Users\flo\git\joomla-cms-5.1-dev\index.php:0 [03-Mar-2024 10:21:56 UTC] PHP 2. require_once() C:\Users\flo\git\joomla-cms-5.1-dev\index.php:32 [03-Mar-2024 10:21:56 UTC] PHP 3. Joomla\CMS\Application\CMSApplication->execute() C:\Users\flo\git\joomla-cms-5.1-dev\includes\app.php:58 [03-Mar-2024 10:21:56 UTC] PHP 4. Joomla\Application\AbstractApplication->dispatchEvent($eventName = 'onAfterRespond', $event = class Joomla\CMS\Event\Application\AfterRespondEvent { protected $name = 'onAfterRespond'; protected $arguments = ['subject' => class Joomla\CMS\Application\SiteApplication { ... }]; protected $stopped = FALSE; private ${Joomla\CMS\Event\AbstractImmutableEvent}constructed = TRUE }) C:\Users\flo\git\joomla-cms-5.1-dev\libraries\src\Application\CMSApplication.php:349 [03-Mar-2024 10:21:56 UTC] PHP 5. Joomla\Event\Dispatcher->dispatch($name = 'onAfterRespond', $event = class Joomla\CMS\Event\Application\AfterRespondEvent { protected $name = 'onAfterRespond'; protected $arguments = ['subject' => class Joomla\CMS\Application\SiteApplication { ... }]; protected $stopped = FALSE; private ${Joomla\CMS\Event\AbstractImmutableEvent}constructed = TRUE }) C:\Users\flo\git\joomla-cms-5.1-dev\libraries\vendor\joomla\application\src\AbstractApplication.php:99 [03-Mar-2024 10:21:56 UTC] PHP 6. Joomla\Plugin\System\Debug\Extension\Debug->onAfterRespond(class Joomla\CMS\Event\Application\AfterRespondEvent { protected $name = 'onAfterRespond'; protected $arguments = ['subject' => class Joomla\CMS\Application\SiteApplication { ... }]; protected $stopped = FALSE; private ${Joomla\CMS\Event\AbstractImmutableEvent}constructed = TRUE }) C:\Users\flo\git\joomla-cms-5.1-dev\libraries\vendor\joomla\event\src\Dispatcher.php:454 [03-Mar-2024 10:21:56 UTC] PHP 7. Joomla\CMS\Autoload\ClassLoader->loadClass($class = 'Joomla\\Plugin\\System\\Debug\\DataCollector\\InfoCollector') C:\Users\flo\git\joomla-cms-5.1-dev\plugins\system\debug\src\Extension\Debug.php:286 [03-Mar-2024 10:21:56 UTC] PHP 8. Composer\Autoload\ClassLoader->loadClass($class = 'Joomla\\Plugin\\System\\Debug\\DataCollector\\InfoCollector') C:\Users\flo\git\joomla-cms-5.1-dev\libraries\src\Autoload\ClassLoader.php:59 [03-Mar-2024 10:21:56 UTC] PHP 9. Composer\Autoload\{closure:C:\Users\flo\git\joomla-cms-5.1-dev\libraries\vendor\composer\ClassLoader.php:575-577}($file = 'C:\\Users\\flo\\git\\joomla-cms-5.1-dev\\plugins\\system/debug/src\\DataCollector\\InfoCollector.php') C:\Users\flo\git\joomla-cms-5.1-dev\libraries\vendor\composer\ClassLoader.php:427 [03-Mar-2024 10:21:56 UTC] PHP 10. include() C:\Users\flo\git\joomla-cms-5.1-dev\libraries\vendor\composer\ClassLoader.php:576 [03-Mar-2024 10:21:56 UTC] PHP 11. Joomla\CMS\Autoload\ClassLoader->loadClass($class = 'Joomla\\Plugin\\System\\Debug\\AbstractDataCollector') C:\Users\flo\git\joomla-cms-5.1-dev\plugins\system\debug\src\DataCollector\InfoCollector.php:32 [03-Mar-2024 10:21:56 UTC] PHP 12. Composer\Autoload\ClassLoader->loadClass($class = 'Joomla\\Plugin\\System\\Debug\\AbstractDataCollector') C:\Users\flo\git\joomla-cms-5.1-dev\libraries\src\Autoload\ClassLoader.php:59 [03-Mar-2024 10:21:56 UTC] PHP 13. Composer\Autoload\{closure:C:\Users\flo\git\joomla-cms-5.1-dev\libraries\vendor\composer\ClassLoader.php:575-577}($file = 'C:\\Users\\flo\\git\\joomla-cms-5.1-dev\\plugins\\system/debug/src\\AbstractDataCollector.php') C:\Users\flo\git\joomla-cms-5.1-dev\libraries\vendor\composer\ClassLoader.php:427 [03-Mar-2024 10:21:56 UTC] PHP 14. include() C:\Users\flo\git\joomla-cms-5.1-dev\libraries\vendor\composer\ClassLoader.php:576

Fixed in #42945. Thanks for the report.