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:
Reduced test case: https://jsfiddle.net/d25ezevc/1/

1. Focus the editable content, notice how spelling errors are highlighted with a wavy underline
2. Click the spell checking toggle button to disable spell checking
3. Focus the editable content again, note how spelling errors remain highlighted. Newly typed spelling errors will not be highlighted, but existing ones remain.

What is the expected behavior?
Once spell checking is disabled, spelling errors would no longer be highlighted

What went wrong?
Spelling errors remained highlighted after spell checking was 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 may not be considered a bug, if not clarification on expected behavior will allow me to push back on my own QA team who have raised this as an issue for me!