Icons, fonts and question marks

My KWGT widgets that include fonts and icons are working only half of the time. They work for a while, then instead of displaying fonts and icons correctly there are question marks instead of icons and instead of proper fonts the widget displays I guess system font. It can change from proper to wrong in 10 minutes twice. Then it can work for hours or not work for hours. I'm using fonticons (fontawesome), Galaxy S8, Android Oreo, Nova Launcher. Here's how it looks: https://www.dropbox.com/s/sagdz682r91vw4l/Screenshot_20180701-221612_Nova-Launcher.jpg?dl=0

The image on top is how it should look and the bottom one is how it looks when it's broken.

The widget is not from SD card. I don't have any app cache cleaner.

What could be the problem here?