I have two designers opened on my computer. One pointing at my development platform and one pointing at my production platform. Both are running 8.0.16. On the dev platform I am able to use pretty much all material icons. Example: this navigation button using material/insights icon. 
On the production platform, this same button shows up as follows: 
Any idea what is going on?
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Interesting observation. When I load the client view from the production platform on Chrome, all the icons show correctly. When I load the same on Brave (Chromium) the same icons are missing that are missing from the designer.