I noticed this recently and last night as well. If you FF or rewind some amount of time (I don’t have a specific measure) and then resume playback, you don’t get the regular source audio. I’m getting a loud, harsh buzzing noise that sends me scrambling for the mute button the remote.
I’m running the latest beta on an ATV 4K and then HDMI into a Marantz AV7704. I’m posting here and not in the beta forum because it happened on prior releases as well.
Stopping playback and then starting again restores the proper audio. I’m primarily playing MKVs of BDs and UHD BDs. Auto set to decode lossless and output as 7.1 PCM.