As an update, this seems to be a Volumio-specific issue.
I’ve tested using other systems / platforms to access Tidal through their own UI, and they work with correct results, with albums available in all their hi-res FLAC versions with no issues. And this is without using Tidal Connect - which also works.
Please help. @volumio
Update 2:
While browsing the forum for solutions I came across another thread reporting similar behaviour (see here: myVolumio only plays Tidal at 44.1 KHz - MyVolumio - Volumio Community )
As per @wheaten’s and the community’s suggestion, I changed Tidal’s Sound Quality settings from highest possible to hi-res under Sources / Tidal / Sound Quality, and that seemed allow for Hi-Res playback again. Problem solved.
Now my question is: why did this get triggered randomly on my system? I had been able to listen to hi-res from Tidal before, even earlier today, using “Highest Possible” as the audio quality setting. Any ideas as to what could be at fault?