If you go to Settings > Cellular > Cellular Data Options > Data Mode, and switch to "STANDARD" instead of "Allow More Data on 5G" the Audio Quality settings in Music separate Wi-Fi and Cellular Streaming

Hi all, just got this set up with my 2018 iPad Pro and ifi iDSD Diablo, and decided to give it a whirl before I went to bed. So far so good. I wish they had a playlist or something that showcased hi-res files - I've just been going through some albums seeing if I can find anything to test out...