@mjg59 Try using an old HDMI cable?
@PCOWandre @mjg59 Seconding this. Had it happen where I used an older cable where it was shorter trying to cable up a soundbar and it didn't support the pins for ARC at all.
@Stormgren @PCOWandre I otherwise need it to support 4K HDR, so I'm not sure an old cable will work…