HEVC with AAC Audio Transcoding DCA to AAC on iOS App



I have an HEVC file in an MP4 container with an AAC-LC stereo audio track. The file will direct stream video and audio to a Chromecast Ultra, however, if I playback the file on an iPhone 6s Plus (iOS app v4.27 on iOS 11.3), video is direct play and audio is "Transcoding DCA to AAC". Any ideas as to why the server is misidentifying the audio track and transcoding when using the iOS app?