I’m not sure we can help you. (@sam_nazarko may have other ideas). It could be some quirk of signal timing between vero and the AVR which would be very difficult to track down. Have you tried different HDMI cables, using a different HDMI input on the AVR and/or a squirt of contact cleaner/lub in the HDMI plugs and sockets?
As I say, the pragmatic answer is not to use passthrough and save up for a modern AVR. I don’t think we’ve had other reports of these symptoms, but correct me if I’m wrong.
Edit: Maybe we need to add your AVR to the list here DTS-HD-MA not always recognised on Denon and Marantz but no problems with TrueHD reported there.