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.
Moreover, there are different quality tags on the WiiM App. The HD tag means the original content is in 16 bit/44.1k while the Ultra HD tag indicate the content is in the resolution 24 bit/44.1k or beyond. Please select the music with 24-bit, 96kHz or 192kHz to try.
Lastly, please check the setting in your Amazon Music setting on the WiiM App. Please select Browse tab > Amazon Music > Settings > Amazon Music HD/UHD. It should be enabled by default.
If you still have issue with Amazon HD/UHD, please send us a feedback via WiiM App by selecting Settings tab > Feedback.