CSS code missing?

I went to change a setting in my css under “WordPress Add Custom CSS” and found it empty, although it seems like the code I used to modify visual elements is being executed. I checked multiple sites I’ve used it on, and same on all. I feel like I’m in the twilight zone. I have been updating these sites to WP 5, installed the “Classic Editor” plugin to avoid the confusion with the Gutenberg editor since I have other page builders in use.

I see others have had some issues after wordpress update. Might this be one as well? Another post asked where the file this plugin generates is located. The answer was the “main wordpress folder”, but I’m not sure what is meant by that. What is the filename so I can find it?

Thanks.