I somehow managed to get a real answer from Pro-Ject support this time - see below.
Apparently, there are so many peculiar things about the hardware around the CM3, that have given up on implementing Tidal Connect:-(
It seems a little strange to me - I thought the CM3 was an entire computer ‘just’ added network and I/O devices. But I’m not that much into putting drivers into a Linux…
Now, I will ask, if they may make some update, improving the search in the embedded Tidal - it seems to me, that it is better by far on a ‘real’ Volumio system.
As a note, we also have an amp from Electrocompaniet with built-in streamer, that supports Tidal Connect.
Their support has confirmed, that they are working on an update allowing Tidal-Connect to play Hi-Res FLAC.
Probably, I’ll end up using the Pro-Ject to play MQA (as long as Tidal makes it possible), and maybe redbook sometimes.
"Unfortunately, the Stream Box S2 Ultra won’t receive a Tidal Connect update. In fact, we have already tried, and ran into issues during certification.
The hardware being entirely custom made, is and always has been a little bit trickier to work with for the team at Volumio and we weren’t able to get it past all stages of Tidal Connect certification.
Sorry to not have any better news, and let me know if you have any more questions."