On your iphone, are you using the tidal app, or some other app? Have you checked the poly FAQ https://chordelectronics.co.uk/faqs-category/streamers/ ? Near the bottom of the page it mentions 'AirPlay is natively limited to transmitting 16bit 44kHz audio. If you wish to playback high resolution files please use DLNA/UPnP/MPD or Roon.' Maybe that is the root cause of your problem.