Cron processed only one item when ultimate_cron installed

Created an MR. The actual logic for processing the items is almost the same with a few improvements:

• Properly inject services.
• Reduce unnecessary indentation for better readability.
• Directly return the number of items processed since it is known.

Thank you for the patch for this specific issue that I've also run into. However after applying the patch I'm getting this error:

Error: Class "Drupal\mailchimp\Queue\Processor" not found in Drupal\Component\DependencyInjection\Container->createService() (line 259 of /code/web/core/lib/Drupal/Component/DependencyInjection/Container.php).

I've cleared the caches. Please help.

Could we please get this committed (once the conflicts are fixed)?

A lot of sites use Ultimate Cron.

This is a bit over my head to fix, to be honest.