But I did find a few that clearly might have been uploaded to Deezer as 320K MP3s with their 20.5kHz cut-off, but based on replies on Reddit, it is sometimes just mastered that way, "Often for dumb reasons". However, sometimes it shows FLAC download unavailable for certain few songs, so maybe it really was just different mastering in those cases.
I end up downloading these as webm, and exporting the audio to MP3 in Audacity. It's a bit annoying for remixes that aren't put on SoundCloud by the original uploader, and the audio quality difference can be noticeable sometimes โน๏ธ