Icons began to disappear recently from the editor, I tried hard refreshing (Ctrl+F5) and even using different device, same results. The issue appears to be caused by connection timeout which indicates that CDN hosting the emoji is down, and CDN appears to be part of third-party MaxCDN rather than Glitch itself. From what I read, MaxCDN got shutdown recently in favor of StackPath.
For future reference, I would highly recommend Glitch to not rely on third-party CDN for content to not have inconveniences like this.