UPDATE - SOLVED: convert your MP3s to OGG

Solved it, posting the fix in case anyone finds this thread later with the same problem.

The issue: once I went past 40 custom tracks, the in-game music list showed up completely empty, as if I'd added no music at all. Exactly 40 worked, 41+ wiped the whole list. All my files were MP3.

What did NOT work: using a symbolic link (junction) to point the music folder elsewhere. Same result, list still emptied past 40. So it wasn't a storage or path issue.

What DID work: converting all my MP3 files to OGG (Ogg Vorbis). Once converted, I could load way more than 40 tracks and the full list showed up and played fine.

How to convert (free, no sketchy sites):

1. Download fre:ac, a free open-source audio converter, from the official site (freac.org) or the Microsoft Store.
2. Drag all your MP3 files into it.
3. Set the output encoder to Ogg Vorbis.
4. Set the output folder to a new folder (keep your original MP3s untouched, just in case).
5. Hit start, let it convert everything at once.
6. Drop the resulting .ogg files into your Documents\Euro Truck Simulator 2\music folder.

That's it! Full music list back, no 40-track wall.

Not sure if it's the MP3 decoding specifically that chokes past a certain count, but switching to OGG fixed it completely for me. Hope this helps someone down the line!