I have observed this too. I have two Elgato Eve devices (door sensor and wall switch) located about 50 feet from the ATV4.
Yes, I understand that Bluetooth range is typically advertised as 30 feet, but even with my extreme placement, these two devices were working with 99% reliability. Even my WiFi accessories have never been 100% so I was perfectly satisfied.
Immediately after the upgrade to tvOS 12, these two devices (and ONLY these two devices) have permanently lost the ability to communicate to the ATV 4. They still work with the Elgato app when my phone is in proximity, but remote access has been lost.
I know I cannot complain to either Elgato or Apple because I am definitely outside the officially supported range, but the fact remains that tvOS 11 was able to make it work, and tvOS 12 reduced the range.
I am just hoping for a point release that may restore things to how they were, otherwise the only option would be to get another ATV just to act as a repeater.