Not sure if this is a server of player issue but since I noticed it on my chromecast I'll post it here.
I was playing a TV show from my Phone (Android 7.0), it was transcoding both the audio (aac 6 channel) and video (mkv) without any buffering issues. At one point the audio dropped out. After stopping and starting again it was ok for a bit and then the audio skipped ahead and was out of sync with the video.
My plex server is version 1.4.1.3362 and I just saw this after updating.