I just updated XD to V184.108.40.206 x64. I'm on a Mac with Big Sur OS.
I can easily create a text field and edit the text no problem. But if I want to change the size of a particular sentance or word or even the color it changes the entire text box and I cannot get around it no matter what.
Thanks for reaching out. I tested this on macOS and latest version of XD and was unable to re-produce the issue. So could you please share a small video of the workflow of what exactly is happening at your end? We'll try our best to help you with this.