According to reports on June 20, ONEUS's maknae Xion recently suffered an arm injury during practice. 

While it's unknown which arm the idol specifically injured, he was reportedly recommended rest while wearing a cast in order to ensure a speedy recovery. As a result, his label is most likely considering his absence during ONEUS's upcoming appearance on 'M! Countdown' this June 20. The group will take the stage as 5-members until Xion's arm recovers fully. 

Get well soon, Xion!