- I happened to drop an album with missing track numbers via File Sharing (iTunes).
- In Vox, the tracks showed with alphabetical sorting (which is logical).
- I deleted the album, fixed the tags and copied it again.
- The tracks still appeared with alphabetical sorting.
- I thought that there might be caching, deleted the album, renamed the files slightly and copied it again.
- Now the proper track numbers showed, as expected.
So, I kinda solved my problem, but in a way that feels wrong. I searched the app settings and wasn’t able to find a way to rescan local library, or parts of it.