Just heard from WP Support
(Added) If you read this, please leave me a comment and let me know what the comment box and the text you type, looks like. And please let me know what browser you use.
I’m still working with WP support to try and figure why some people can’t see what they’re typing in the comment box.
I contacted WP Support several days ago, and told them about the problem some of you are having with the comment reply box.
They suggested clearing out your cache and cookies to try to fix that problem. Here’s a link that tells you how to do that:
Clear your cache and cookies
Good luck! Hopefully this will help, and you can see what you’re typing again.