The missing ones here are because I switched to "Native" style temporarily so you'll only see the emojis that your device properly supports. The missing ones look like the very latest ones. Not sure about Windows 10 but certainly for iOS they wouldn't appear properly until iOS 12.1.
But we've now switched the emoji style back to "EmojiOne" so it should look better?
We have made several changes here to improve speed in a few ways and made a few tweaks to the "native" emoji style. However, unfortunately native emojis will potentially always have some issues like this as it's totally unknown what exactly is supported.
If you have any other issues related to this, please start a new bug. Please test immediately before reporting though, as we're rolling out changes essentially constantly.