The short explanation
Dolby TrueHD is losslessly compressed: it takes less space in transit but decodes to exactly the original audio, unlike Dolby Digital which discards data permanently. That makes it demanding on bandwidth, which is why it needs HDMI rather than optical.
On Blu-ray and UHD Blu-ray, TrueHD is also the usual carrier for Atmos. The Atmos object metadata rides inside the TrueHD stream.
Where you run into it
Which means losing TrueHD often means losing Atmos. If the capability data reaching the player does not list TrueHD, the player falls back to PCM or a Dolby Digital track — and the Atmos layer goes with it, silently.
Over ARC, plain ARC has nowhere near the bandwidth for TrueHD; eARC does.
How the DR HDMI 8K handles it
TrueHD is one of the audio flags in the DR HDMI 8K's EDID controls, with its own mode and sample rate settings. You can set it or disable it — which makes it straightforward to test whether a missing Atmos track is a capability that was never offered.
Want to test your own HDMI chain?
The DR HDMI 8K is sold through the HDFury.eu web shop.