I changed back and forth between Passthrough and Downmix over the past couple of days. Downmix always leads to a drift of the audio to lag behind video (all the latest updates installed).
Passthrough keeps audio and video synced and toggling Downmix and Passthrough brings audio in Downmix back to sync for a while. What a strange issue!
I have permanent time shift activated and use it frequently. I have the impression that using it could increase the lag.
I will now try a fresh install from scratch and see what happens...