[ryerman]

Hi Petrz
I'm sure Winamp writes to MP4 files and that the tags persist after the file is removed and re-added.

Attached are 4 screen-shots to show what I see on my system.
2 show a file in Winamp. Another shows the same file in the details view of Windows 10 File Explorer.
Take note that not everything that is available in the details view is shown in the right-click file properties dialogue.

The 4th screen-shot shows the file in Exiftool.
Exiftool is an application that reads metadata for many file types.
Apparently, Winamp writes Quicktime tags.