https://www.cnet.com/news/apple-releases-iphone-update-to-fix-group-facetime-eavesdropping-bug/

Release notes for iOS 12.1.4 say a "logic issue existed" in Group FaceTime and that it was fixed "with improved state management." As of 10 a.m. PT on Thursday, Apple's System Status page notes that Group FaceTime service has been restored. 

In addition to the Group FaceTime bug, Apple said it also fixed a previously unknown issue with the Live Photos feature of FaceTime.