On searching the Roon forum of the limited 2 channel Tidal functionality on Roon I found this response..
The MP-50 does support multichannel PCM audio over HDMI. I use an Intel NUC running Roon Bridge on Ubuntu as my player. Works fine. Multichannel audio input clearly shown on the MP-50 Web interface.
So, if any Lyngdorf owner is willing to experiment with a direct HDMI connection between their Roon server and their MP-40/60, we can ascertain whether multi-channel Atmos music playback is actually possible directly from Roon/Tidal.