This is still an issue for me as well. I can't believe they haven't fixed it yet, and I'm disappointed to hear that the people who have called support are being told the issue can't be replicated. It makes me believe nobody on their end is realizing or admitting there is a problem.
The biggest issue I have is that occasionally the Squarespace editor simply slows down - so that the letters can't keep up with the speed of your typing. It's incredibly frustrating and makes work impossible. Strangely, it doesn't happen every time, but none of the workarounds in this thread have worked for me when the issue does happen. It has nothing to do with your computer or your browser or your internet, it has something to do with Squarespace's software or their servers or something on their end, and it absolutely needs to be fixed.
For me the only solution is to type in a word doc and paste it over to Squarespace a little bit at a time. Sorry I can't be of more help, but hopefully if more people complain, actual resources will be used to fix this.