Same issue here.
When I am downloading all the MaterialIcon fonts files from the github (on master) on my server and refering to them in my font-face url, the font does not load but when using the one provided by gstatic (https://fonts.gstatic.com/s/materialicons/v36/flUhRq6tzZclQEJ-Vdg-IuiaDsNc.woff2), it works fine.
Hey @CharlieMcFly @jonbodart
This project is on low maintenance and out of date for a while.
The lack of updates + development support issues also bothered me. I've forked this repo, added support for scss, removed irrelevant files and updated the fonts with the latest straight from Google's CDN.

Most helpful comment
Same issue here.
When I am downloading all the MaterialIcon fonts files from the github (on master) on my server and refering to them in my font-face url, the font does not load but when using the one provided by gstatic (https://fonts.gstatic.com/s/materialicons/v36/flUhRq6tzZclQEJ-Vdg-IuiaDsNc.woff2), it works fine.