Verified answer

If you're using a laptop, I suspect the mouse touchpad is so sensitive it's picking up your thumbs or palms even though they're not touching it. Look for a switch on the laptop or a mouse option to reduce its sensitivity or turn it off and see if that helps.

Hope it does.

When Word alterations the letter you must get a bit of image on the part of it (slightly like a lighning bolt). If you do, click on at the drop down arrow ( which looks whilst you hover on it) and decide on the choice to quit replacing the case routinely. Otherwise you'll be able to move into, Tools, Options and switch it off that method. Good Luck