Some more info and an update:
I’m using Longform with some plugins that don’t mess with post formatting at all. I have Aesop active, but I’m not currently using it on any post or page.
I created a child theme with Osorbius Child Theme Creator and use the Osorbius editor instead of the built-in CSS editor. Any CSS entered into this editor and saved in your child-theme will automatically be applied after the parent theme styles.
Any paragraph I want to have the drop-cap I gave the class “drop”
Now I just put the CSS into the editor and it works. Instead of “.entry-content” above (which applies style to every first paragraph on the site) I use “.drop”