"Outro" was the group's last album after the members enlisted in their mandatory military service, with Yoon Doojoon enlisting on Aug. 24, 2018, and was discharged on April 10, 2020. On Jan. 24, 2019, Yang Yoseob enlisted in his military service as a conscripted police officer and was later discharged on Aug. 30, 2020. Lee Gi Kwang enlisted on April 18, 2019 as a conscripted police officer as well, getting discharged on Nov. 17, 2020. Son Dongwoon was the last of the group to enlist, entering the military on May 9, 2019 and being discharged on Dec. 7, 2020, marking the end of all Highlight members' mandatory military enlistment. The highlight, which started with the enlistment of leader Yoon Doo-jun in August 2018, and celebrated the military white flag, became'Military Pildol', who completed the duty of national defense by the youngest son Dong-woon in December last year.