I suspect there is some bugginess in the Musiccast sync settings. I have 4 music cast devices and use my 3060 as the master often to run the other devices. I have never changed the sync, but it sometimes ends up on the lip sync setting anyway.
The default sync is audio which keeps all the devices in sync for music, but if you are watching video it will be out of sync.
Be sure to go to the Musiccast app and uncheck all the connected devices and hit OK before going back to video on your AVR.
That puts everything back to normal for me.