The menu is positioned differently on desktop (position: fixed) vs mobile (position: absolute) so because of 
CSS magic 
the overflow-x: auto; on the .d-editor-button-bar container was hiding the menu on mobile but has no effect on desktop.
That overflow-x: auto; property fixes this issue on desktop:
I just added a fix that shows the menu again on mobile by removing overflow-x: auto;
https://github.com/discourse/discourse/commit/d8714facb605e1ec8fc5f2021b2551a0fce51e18
We’ll have to figure out if we can change positioning or do something else to solve the toolbar overflow issue on mobile.