The Lamanites did indeed march forth and attack Cumeni, seeing that Gid and many soldiers had left for Zarahemla. But Gid returned just in time to help Helaman’s band win the day. Heavy losses were suffered, but not by the stripling warriors.
Book of Mormon Central, “Why Did the Stripling Warriors Perform Their Duties ‘With Exactness’? (Alma 57:21),” KnoWhy 165, (August 15, 2016).