Roon Signal Path can be very revealing. I was playing some jazz albums from Tidal and suddenly I noticed that the signal path light was yellow. I had never seen that before!
Yes there are a few MP3 albums in there. In tidal the HIFI indicator goes grey when they are MP3. However, I have noticed that some previously MP3 albums are now HIFI. Good on tidal, they are upgrading their catalogue where possible it seems.
In all honesty there are very few, I went though and logged some of my albums in my collection via Tidal and I counted about 6 in 1200 which were not HIFI, ie MP3
Can it be that the label, at least during a period, only supplied Tidal with MP3s? Sound very strange, but if that’s the case I am glad they didn’t convert to another lossy format.
I have not read everything in this thread. But a good approach if you find MP3 albums in Tidal Hifi; contact Tidal and complain. Those I have found have been fixed (new version in FLAC were made available).