Numbered content configuration is persisted in CKEditor fields when saved too fast

Reproduction steps:

• make sure CKEditor is the default editor
• edit a page with CKEditor fields (e.g., the current user profile)
• edit the page
• save the page before the CKEditor fields are fully loaded (the network speed can be throttled in the web development tools of your browser for a slower CKEditor loading time)
  Expected:
• The CKEditor fields are saved with the same content as before the edit
  Actual:
• Some junk content apparently coming from numbered headings configuration is persisted in the CKEditor fields