WiiM Mini supports the bit perfect output such that your audio output will match the resolution of your input source. Even though the SPDIF output resolution is set to 24 bit/192 kHz, it won't up-sample the audio to the fixed resolution of 24 bit/192k Hz. Please check the sample rate and bit depth of the played music on your native Amazon Music App and see if it matches with the output from WiiM device.
Please select Browse tab > Amazon Alexa > Settings and turn on Amazon Music HD/UHD on WiiM Home app.
If you still have issue with Amazon HD/UHD, please send us a feedback via WiiM App by selecting Settings tab > Feedback.