It was configured to clean on Browser Quit.

I recently upgraded from Mojave to Catalina. Since then, I've seen some alerts on login related to making changes while the browser is running being a bad idea. (Sorry I can't remember the details.) After the most recent such incident, I started Safari and it had no tabs or history. I restored some missing ~/Library/Safari files from a Time Machine backup, and Safari appears to have regained its memory. I can't say for sure whether these two issues are related, but it seems possible. I'm using Cookie 6.1.2.