I've had the AMP in my own home since summer doing beta testing and haven't experienced a single hiccup with ARC using the new AMP with a 2017 Samsung QLED.

We only have the TV on maybe 2-3 hours a day, but the use is mostly my wife and son who would both let me know immediately about any problems.

I did do some quick testing using one of the latest SnapAV HDBT extenders that supports ARC and it was able to return audio from the TV back to the AMP.  This was all done very quickly and temporarily with CAT5 cable strung through the house across the floor.

For those of you wanting to use HDBT for ARC with the new Sonos AMP I'd encourage you to contact your HDBT manufacturer/distributor of choice and inquire if they've done any testing with the new AMP.