I recently bought a new AVR for the dedicated HT room, so connected my 2.5 years old Pioneer Elite AVR with Q1 75". HDMI with ARC seem to work flawlessly between the TV and AVR including CEC controls like Volume adjustment, auto power on/off. Also the TV's remote works well with the Fire stick 4K except for the home screen. Pressing on TV remote takes me back to TV's home screen while in Fire stick's HDMI port. I think that is understandable.
But I couldn't get audio on the AVR over HDMI ARC while playing content from any of the built-in apps on the TV. The AVR shows TV on the display and volume control works over CEC but no audio on TV/AVR.
Not sure if I am doing something wrong. But ARC works well with Fire stick and I typically don't use TV apps. So it is not a completely hopeless situation.