I think this might have to do with apostrophes and cursor placement? I've just had this occur again when typing "that's ". It seems once the bug starts all the text is affected.
Edit: this issue isn't exclusive to the rich text editor. I can reproduce it on profile post comments. Perhaps a SwiftKey issue?
Edit: this issue isn't exclusive to the rich text editor. I can reproduce it on profile post comments. Perhaps a SwiftKey issue?