Some file types copied onto phone always disappear

Question

Whenever I try to copy files onto my phone, depending on file type they almost immediately disappear. I noticed this when trying to copy music files (mp3 and flac) from my PC to the phone via the USB cable, using total commander.

• What works: png, doc, pdf, xlsx

• What gets deleted: mp3, flac, wav, somefile.someextension

I copy the file(s), and they disappear. Sometimes it happens immediately, sometimes it needs a few seconds, a couple of refreshes, or navigating away from the folder and back in again. Sometimes I can see the file from my PC but not on the phone screen for a while (up to a minute). But the end result is always the same, the files soon disappear.

I don't think the files are being moved anywhere on the phone, as I tried searching the entire phone for the missing files and found nothing (no mp3 files on the entire phone), while I can still find other file types just fine.

The phone only has internal storage, no SD card. I can browse its contents just fine from the PC, and like I said this only affects certain file types.

I've also tried copying from a USB stick (one that has a mini-usb on one end so it plugs into the phone), and I also tried several file managers to do the copying, the results are always the same.

I tried copying to the Music folder, and to another custom folder, no change in behavior.

I tried adding a .nomedia file to the folder I'm copying to, it didn't help.

I tried using MTP or PTP connections.

I don't know if any apps are doing this, but I tried disabling the built-in "Files by Google" app, and stopping all other apps. I really don't have a lot of apps on this phone, and I have no idea which one could be doing something like this and why.

The only somewhat relevant question I found was this one , but that's about bluetooth, and I'm using a USB connection to a PC.

Please someone let me know how to use a modern smartphone to provide the functionality of a fifteen year old mp3 player.

Edit:

I did a factory reset, then tried developer mode and adb. I tried both before and after the system has updated.

ls doesn't show any of the files that I tried to copy. Here is what I think is the relevant part of adb logcat while copying a file called Gemini.mp3: https://pastebin.com/bmTaLfgk

Basically there are a couple of these:

Access denied finding property "media.stagefright.extractremote"
Access denied finding property "media.metrics.enabled"
Access denied finding property "media.metrics.enabled"

and these:

mediaserver: type=1400 audit(0.0:524): avc: denied { read } for name="u:object_r:media_prop:s0" dev="tmpfs" ino=8498 scontext=u:r:mediaserver:s0 tcontext=u:object_r:media_prop:s0 tclass=file permissive=0
Binder:1257_2: type=1400 audit(0.0:525): avc: denied { read } for name="u:object_r:media_prop:s0" dev="tmpfs" ino=8498 scontext=u:r:mediaextractor:s0 tcontext=u:object_r:media_prop:s0 tclass=file permissive=0
Binder:1257_2: type=1400 audit(0.0:526): avc: denied { read } for name="u:object_r:media_prop:s0" dev="tmpfs" ino=8498 scontext=u:r:mediaextractor:s0 tcontext=u:object_r:media_prop:s0 tclass=file permissive=0

And then something called MediaSorter decides to remove the file.

MediaSorter: Removing /storage/emulated/0/Music/Gemini.mp3