CiviCRM CK editor trying to access Skins.js, and CSS resources from the Drupal 8 core assets instead of the Civi directories
I also had this problem.  It happened ~95% of the time but not 100% of the time, leading me to believe there's a race condition present.  It started when I upgraded from Civi 5.12.1 to 5.12.3, but you experienced this problem sooner and I'm guessing it's tied to Drupal 8.6.15.  It also happened on my test site but not my dev site (haven't pushed to the live site yet for obvious reasons).

I was able to solve it by uninstalling/reinstalling the core ckeditor module via drush.  However, I'm not sure why that fixes it, but I've been unable to trip the issue.  In my debugging, it certainly seems that Civi's CKEditor is loading the paths of the Drupal CKEditor.  I'm not sure why though.