Same reason.
Keep in mind that Tidal is a lossy format, consisting of to parts.
Very simple explained
part one is the audio lossy container, part 2 is the gimmick box (MQA).
Inb order 2 use the gimmick box, the device must be licensed by MQA. Since you PC is not licensed, it will only play the lossy audio container. It passes the same information to your DAC.
If you use a program like JRiver on your PC (and disable all audio signal manipulations), you can also pass the full signal to your DAC.
Seems the BT connection is passing the signal in full.
See also: Volumio 3 web interface shows 44.1kHz 16bit but - #11 by Wheaten