Have you both looked into the group delay correction that's available in settings under each speaker's "Audio" heading in the Home app? I'd also take a look in your TV settings, there may be an audio sync adjustment there as well after all sound is processed there before being sent out to the speakers. Don't know if it will fix it or not, but the option is there and that's what it's supposed to do. Bluetooth may work flawlessly because it has lower sound quality to begin with, not to mention the secondary compression that happens in order to transmit the file when compared to WiFi.
Sound sync issues can be caused by any number of things, the broadcast signal, interference with the method of connection, audio processing delay (internal to the device sending the signal) or even issues with pulse code modulation (the process of converting analog audio to digital audio). Not to dodge the issue but, the Nest speaker simply receives what it's sent; processing happens upstream. The adjustment in settings may, hopefully, be able to correct and compensate for all that.
