After doing some research on my phone (Xiaomi Mi A1), it seems that it indeed does NOT support Hi-Res. It's processor is Snapdragon 625 which DOES have Hi-Res supports due to a Qualcomm Aqstic audio codec. BUT, for some reason, Mi A1 DOESN'T HAVE Aqstic, it's just not there.. instead audio goes out through some msm8916, and as a result, the phone only supports 16/48 (which can be confirmed by both adb command AND system logs). So, unfortunately, Mi A1 doesn't and can't support Hi-Res.
Why does Neutron show that it does? Probably because it sees the processor, which is supposed to support Hi-Res, but it doesn't (not when installed in Mi A1, looks like when creating the phone they cut some corners.. Mi A1 also doesn't support Quick Charge 3.0, aptx, and other stuff that it's supposed to)..
So, do I understand correctly, that since my phone DOESN'T supports all frequencies shown in Neutron with Hi-Res Codec (Direct PCM), I should instead use Custom Format and install 16/48 (what is shown in audioflinger and system logs)?