I’ve spent some time debugging these tinymce issues and believe to have found a fix!
If possible, can you please download and test the latest build of ACF PRO (5.4.3 RC1)?
1. Login to your account online: http://www.advancedcustomfields.com/my-account
2. Click on the ‘see all versions’ link next to your license
3. Download the latest 5.4.3 RC1 version
Please let me know how you go. I’ll be able to release this as a public update when I hear back form a few other tickets
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