Update: If I re-save the page, the new lines appear correctly in the textarea. I looked in the database and it appears new lines are stored as actual new lines rather than \n. (Screenshot of database: https://ibb.co/p2hRtMK )
So it looks like the real problem is with the way WP All Import is adding \n rather than a new line. I may need to separate these titles into 6 different fields if there is no way around this. Not an ACF problem after all.
