Apple TV third gen used for optical sound doesn't sync any more with Apple TV 4th gen

Hi there I use an old 3rd Gen Apple TV to provide optical audio for my 4th Gen Apple TV.

This was a great hack when the optical port was discontinued by Apple, and I think quite widely used

The Apple TV3 is connected to the Amp with an Optical Cable, and you select it as a speaker from the drop down, which also shows the regular TV (annoyingly set as default and unchangeable) AirPods etc

Since the latest update the sound is now very out of sync. Works fine with AirPods and directly to TV.

Any remedies?