You were correct on #5. For reference, the plugin is called “YouTube Zero Annotations”.
Disabling the browser plugin stopped the code from showing up. I do not know why the plugin triggered on the WordPress Editor but I guess I will just have to remove it. The creator is keeping up with it so I will report this issue to them and hopefully they can figure out what went wrong.
I had forgotten that I even had it installed. Thanks for the help.
It still occurs with all plugins deactivated.
