It happens with tracks using all upload options, I’ve tried every one. VOX Universal (beta), VOX Uploader (beta), VOX Music Player for macOS, and web-upload tools.
Some tracks will be fine and some will show up, then disappear, and some just won’t appear at all. If I force the upload by trying again several times it will eventually work but obviously the service is not working as it should. I have provided error logs to the support team previously. This happens when uploading a single track or multiple so doesn’t seem to be an issue with how many songs are in the queue.
Albums will sometimes upload only some of the songs as well, I have to repeat the process to get all songs uploaded.