It's a bit confusing, but all of these apps (including UAPP running under Android Oreo) are accessing the ESS DAC. The problem is the DAC hardware can't natively process 44/16 files. I don't know why it was designed this way, but I guess it was a cost-saving exercise to have one pipeline that could handle multiples of common hi-res sample rates, e.g., 48, 96, 192, 384 kHz, and tnen one other 24-bit pipelne was needed for MQA, so 44/24 is supported, but not 44/16.
So now, when you play any 44/16 files (FLAC, mp3, AAC, ALAC, etc.) you have two options - up-sample to a supported mode (48/16), or bit-pad.to 44/24. The former causes nasty artifacts in the sound; the latter is totally harmless. So Neutron and UAPP were actually sending your 44/16 files to the DAC chipset as 44/24 streams.
Neutron just needs a few extra toggles set (see updated first post in this thread). I have not yet found a work-around for UAPP under Oreo - your music will still play through the quad DAC, but you'll get the same re-sampling quirks you get from the native LG music app.