Currently when the chalkboard plugin (https://github.com/rajgoel/reveal.js-plugins/blob/master/chalkboard/README.md) is used, two icons (chalkboard and canvas) are added next to the "menu" icon (three bars).
See following screenshot of the icons:
While the "menu" icon is removed on export the two chalkboard related icons remain.
I didn't check how you remove / hide the "menu" icon, but here is a screeshot showing the div that holds the chalkboard icons:
