I had the free trial expired, so I bought a 1 year Pro subscription from the ATV4K Settings (I don’t know if it matters). All movies that were playing fine with audio passthrough (either in DTS or AC 3/EAC3) now they show in the AVR as PCM 2.0. I didn’t change anything in the AVR.
If I use the “Auto” option I get PCM 5.1, but that’s not exactly why I paid for it. DTS works fine from Sony’s Video app on my TV via HDMI-ARC, showing as such in the AVR. I tried power cycling the ATV 4K, AVR and TV. No changes. What changed? I hope it hasn’t to do with the last tvOS 11.3.