I had ARC drop out concurrent with updating to 10.0, not sure if that was actually the cause or not.
Struggled getting it back working properly but did it this way -
Started with 2 devices plugged in to HDMI 1 & 2, receiver into HDMI 3 (arc)
System -> CEC menu showing no CEC devices, Audio -> passthrough shows only Stereo
Unplugged all HDMI from TV -
Plugged receiver into HDMI 2 - now CEC device shows as detected but not in ARC port
Move receiver back to HDMI 3, plug everything back in - all CEC devices showing now in menu and ARC working properly
Maybe that will be helpful to someone else having this problem. Really annoying.