We tried updating the HTML editor’s library (called TinyMCE) to the latest version to see if they fixed the bug you reported, only to find even more bugs!
Even if it is one of the best and more customizable WYSIWYG editors around, Tiny MCE is still not capable to handle all the different use cases, simply because they are very hard to predict and manage! It is a battle that the both of us can’t win.
That is why we created our own WYSIWYG Editor, the Structured Text, that is built around DatoCMS users needs; and that is why we finally decided to stop taking into account support requests regarding the HTML editor unless they concern security updates.
If you don’t feel like using our Structured Text, you can still build your own DatoCMS plugin containing the version/configuration of TinyMCE that most suits your needs.
About your specific case. The bug you see is connected to the fact that Tiny MCE is not able to handle lists inside divs. If you remove the div that wraps the list, you should be able to switch from list to plain text by using the button.
I hope you understand our view about this topic!