Thanks for the bug report.
I have a strong feeling that this issue is resolved in ACF PRO (ACF v5) because the implementation of the WYSIWYG field is much much smarter (and the above highlighted code no longer exists).
Are you able to give ACF PRO a go? If you have purchased an add-on in the past, you will see a free version in your store account
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users