Actually there is no ARC happening when I use Fire stick. I just realised that I connected Fire stick to the AVR, not the TV.Yes, the TV has bug.. a lot of us are facing similar issue. Don't think that MI is going to resolve this. Consider this as a non smart TV and you should be happy.
Luckily the AVR supports DV passthrough. Watched few scenes from some Netflix DV shows. They look stunning. But the same HDR content looks terrible when Fire stick is directly connected to TV or when played from the native Netflix app on the TV. Not sure why.
As you said, I am currently using it as dumb display panel. But I must say I am quite pleased with the PQ and brightness. Hope it lasts for at least few years.