This issue could be related to the way Snapchat works. There haven't been any news regarding the disappearance of users from the lists which means there was an error in the application or you've done something wrong. However, to overcome this, you and your friend should remove the apps, then re-install the new version through the Play Store. Keep in mind, that the Best Friends list isn't available anymore because the company decided to remove it as it raised privacy issue.
