While not an ARC specific thing. I too have had some odd issues with HDMI CEC with my Vero 4K +.
One issue is where the Vero seems to take input control, even when it’s not commanded to. I’ve not been able to pin down how or why.
The other one, which was a new one I experienced yesterday, after updating the firmware on my Yamaha RX-V3083 (aka A3070), when I went to play something on the Vero 4K, it’d play the first second or two, then something would happen to the HDMI visual signal where it’d change resolutions to something incompatible.
I’d have to switch away manually from the vero input, then back to it again, to get the menu to even come back.
It was readily reproducible at that time too.
I was a bit worried that it was related to the firmware update to my receiver, but after rebooting the Vero, it all came good again and has been working fine since.
There’s definitely something a little screwy going on.
Is it too late to capture logs given I’ve rebooted it and its been a day or so?
Edit: actually, the later issue sounds quite like this: