Thanks @whgandalf for the post. I experienced the same issue after an update of the AS (Awsome Support) plugin in WP (Wor Press) 4.9.8
In fact this issue is by the TinyMCE.
After installin the TinyMCE Advanced in my WP, to replace the current editor the issue persit.
So I uninstalled this plugin and then the issue desapeared.