Us old people have the idea of peripherals as input (keyboard, mouse) or output (monitor, printer) - but they've all been input/output for two decades or more; and Thunderbolt and friends are much more like an Ethernet network (reliability of Wake on LAN, anyone?) than they are simple point-to-point connections.
Meaning - anything could be the device preventing what you expect to happen, even if it looks like it should be impossible and have nothing to do with it.
CEC and eARC piss me off to no end - it was all working perfectly for over a year and now the receiver refuses to cooperate (or maybe it's the TV, who knows) and you have to cycle things by hand like a meanderthal.
Meaning - anything could be the device preventing what you expect to happen, even if it looks like it should be impossible and have nothing to do with it.