Neither UAPP nor Neutron do unfolding at 384 kHz on the DX300.
In fact, Neutron does not unfold any MQA at all in the DX300 or any other player, basically because Neutron does not support unfolding MQA.
Does this mean Neutron does not play MQA files? No, of course it does reproduce them. Just like any other FLAC compatible application, it will play them, because what it plays is the FLAC containing the MQA. And it also warns that it is an MQA, but does not do any unfolding.
To check it you have to look for an MQA file saved in a CD-quality FLAC container. For example, this:
If you try this file in Mango, you will see that it unfolds at 384 kHz. If you try it in UAPP you will see that it only does the first unfolding, by software, and plays at 96 kHz. And if you try it on Neutron, you'll see that it plays it like a 44kHz FLAC.
But if you try it with an MQA contained in a FLAC file that is directly at 384 kHz (this for example: http://www.lindberg.no/hires/mqa-2018/2L-145_01_stereo.mqa.flac
), all of them will play it at that bitrate, without doing any unfolding, since the FLAC that contains it is already at that bitrate.
Note the size difference between both MQA files, the first one is contained in a 44 kHz FLAC, while the second one is contained in a 384 kHz FLAC, and it is much bigger.
In an MQA-compliant player like Mango, both should play at the same bitrate and sound exactly the same. That does not happen neither in UAPP (in the DX300) nor in Neutron (in any DAP).