After talking with Matt from CouchLearn to see why he hasn’t encountered the issue, another workaround seems to be to mute and unmute the player in beginplay/endplay events. That also resets the VOIP to take account of the attenuation. Calling RemoveAllRemoteTalkers also seems to reset the VOIP to recognize the attenuation.
So what seems to be happening is if you don’t toggle or call mute/unmute in the beginplay, a player that is already speaking before creates an audio stream that isn’t affected by attenuation that is added perhaps a few frames later in the beginplay call.