Unity 5.6.0f3 and Odin 18.104.22.168
Several of the list manipulation button icons stop drawing when you change scenes. I was viewing your own demo scenes that come with the project. When I open Unity, it appears there is no issue when looking at a list in one of your scenes. All icons are present on the buttons. But if you switch scenes, and look at an inspector list, you will notice icons are missing. And if you switch scenes again, more icons go missing. Only way to get them to show again is restart unity.