I just upgraded to civicrm 5.12.0 and the ckeditor is not available when editing an event. It is there when entering a new event. I think we had this problem with the previous version as well.
My solution to my own earlier question: WYSIWYG editor not present with WordPress may provide an answer. If this was the cause of your previous problem it would have reverted on an upgrade.
Is CKEditor missing in all circumstances (eg sending an email)?