There appears to be a bug with the RadzenUpload single file selection. The bug is very repeatable.
In single file selection mode, select a file, then remove it. The file will clear. Ok, now re-select the same file again. RadzenUpload doesn't detect the change and nothing is added.
While your team is working on this, consider changing the Radzen.FileInfo.Size from int to long. Technically, if you're using the System.IO FileInfo namespace, the file size (length) is long, although that would make a really huge file to upload (even if you are only approaching the int.Max)!!
Unless, of course, your Team did this intentionally to restrict upload size.