During the showcase, Eunjin had shown signs of breathing troubles, after taking medicine on an empty stomach. According to MBK, "Eunjin left the hospital early in the morning [on April 20]. She is feeling recovered, so she will be able to perform on 'M! Countdown'."
You'll be able to catch all 9 members of DIA during their comeback stage with "Will You Go Out With Me" on 'M! Countdown'!