I've found two ways. Both are kind of ugly...
...but first, it's important to note that there are two interface styles for the find action: what you are describing is a mode that is used when the window is rather narrow: a popup at the center of the window asking for the search string.
There's a different mode where the search input is in the toolbar. That mode is used whenever there is enough space in the toolbar.
So, that leads to the first method: make the window wide enough to accommodate the second mode. Because that makes ESC work as expected, and as implied by Marc's incredulous comment.
Here's a second way to do it, in case you are working on a 4:24 aspect ratio portrait mode display, or just feel that preview doesn't deserve those pixels: after entering the find mode, hit COMMAND+E. This will somehow change to the toolbar-style find mode, and you can hit ESC from there.
All tested on Monterey Beta.