(Part solved) Missing line items on event invoice and confirmation email for Event (using Webform CiviCRM)
I believe this PR by Andrei (who works on the Wordpress/Caldera form integration) may help fix this (Core) issue:

https://github.com/civicrm/civicrm-core/pull/13477

It removes bad logic re: participants -originally included in Core invoicing code and works great. I’ve tested it/used it myself using Drupal/Webform CiviCRM module.

Unfortunately Andrei’s PR has been deemed ‘needs work to be reviewable’ - so it has gone nowhere. And that’s unfortunate.