I wish I knew. I spent over an hour a week or so ago painstakingly reformatting pasted text. I ended up having to manually edit the HTML because I couldn't get any of the formatting buttons on the toolbar to work. It's a known fact that Word creates very
messy HTML, but there used to be a button to insert as plain text, and I couldn't get that to work, either. Nor did it help to strip formatting from the text in Word before pasting.
AFAICS, every "improvement" WordPress makes in the editor just makes it totally unusable. I had one page that I couldn't even get to the bottom of before I finally figured out how to hide the Page Option block/widget that was covering it up.
But this is at a website "managed" by WordPress. I don't have the same problem with my WordPress blog site, where I can compose in Word and publish directly to WP.