If you turn around with a keyboard and mouse while sprinting your sprint will cancel.
I see you already have a discussion topic open about this here:
https://www.halowaypoint.com/en-us/forums/f3681fbc78fa43878c4aec9a63a46224/topics/sprint-is-still-broken-on-kbm-halo-4/9b476117-b83d-4a18-b84c-187ec585f2d3/posts?page=1
If you feel this this is a bug and not intended behaviour, please file a ticket on the Halo Support site