UserAgent: Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/66.0.3359.139 Safari/537.36

Steps to reproduce the problem:
Demo of issue: https://jsfiddle.net/d25ezevc/1/

1. Focus the editable div, notice spelling errors are highlighted
2. Click the toggle spellcheck button to disable spellchecking on the editable div
3. Focus the editable div, notice spelling errors remain highlighted until they are modified, but new spelling errors will not be highlighted

What is the expected behavior?
Spelling errors would no longer be highlighted

What went wrong?
Spelling errors remain highlighted after the spell checking has been disabled

Did this work before? N/A 

Does this work in other browsers? N/A

Chrome version: 66.0.3359.139  Channel: stable
OS Version: 6.1 (Windows 7, Windows Server 2008 R2)
Flash Version: 

This might not be considered a bug, but I need clarification on expected behaviour then so I can tell my QA team not to report it as a bug to me!