FontAwesome Icons not showing properly
New font-awesome 5 icons are not showing properly.
Also icons look a bit cramped as well

having the same issue here! 🙈
That's because simplemde is downloading font-awesome. This is happening when using FontAwesome V5 which is now using SVG's instead of CSS + Webfonts.
This is fixed in my fork inscryb-markdown-editor. This project is not longer active per the developer
This happens because it is downloading fontAwesome from a CDN. When you include fontAwesome js and the simpleMDE js files it causes the simpleMDE icons to render improperly. There is an option "autoDownloadFontAwesome" which apparently will look to see if fontAwesome is already included but I cannot get it to work.
seems like simplemde use class of fontAwesome under version 4 in fa-* style, but in fontAwesome 5, the class have changed to fas-*, and the heading and image icon are missing for the mismatch of fontAwesome (autoDownloadFontAwesome set false)

This is fixed in my fork inscryb-markdown-editor. This project is not longer active per the developer
@CWharton
Thank you for forking and fixing. Appreciated.