On January 7, 2020, Blockberry Creative announced that leader HaSeul would not take part in promotions of the next album due to mental health problems. She was said to be diagnosed with “intermittent anxiety symptoms” and would be taking time to take care of her health. On February 5, 2020, Loona released their second EP titled "[#]" (read as "hash"), along with the title track "So What". Although HaSeul did not help with the title track, her vocals are featured on three of the songs on the album, including "365". The EP debuted at number 2 on the Gaon Album Chart, and has become the group's best selling album to date. On March 12, 2020, Loona took their first music show trophy with "So What" on Mnet's "M Countdown".