If icons are missing, there are 2 common issues:

1.) Critical Font feature is not compatible with your theme/plugins

In some cases themes or plugins are using customized version of FontAwesome, which may is not compatible with Critical Fonts feature. Please dequeue Critical Font and check the cached version again.

2.)

If you don’t even use Critical Fonts, or it disabling it didn’t solve the issue, the other common problem, is that pages which are cached with an Internet Explorer or Edge user agent, will load only .eot version of fonts. Please enable Settings > Optimization > General > Optimize Prebuild Only and Settings > Caching > Warmup > Prebuild Cache Automatically in order to load the proper font files.