When I click add a card, the description of the previous card persists. I’m on the latest version on Mac M1 chip and have no add-ons. As you can see in the screenshot, after clicking “Add” the title is gone but the description persists.
Above the Back field, at the extreme right, you see an icon that looks like a thumbtack. Like this: 
, but vertical. You have turned it on by accident. You should turn it off by clicking on it again.
Turning on this icon makes the field a “sticky field”. See the documentation about this.
thanks! rookie move