You've got some user access and permissions issues going on. That's what is causing your problems. Unfortunately I cannot give good advice on how to fix that. I don't know enough about the details of Linux file permissions and how they translate/transfer to how Windows sees those file permissions when Windows accesses those files over SMB or other network file sharing protocols. I don't know the details of what is going on and why.
Hopefully someone with more knowledge about Linux file sharing and how Windows sees the file permissions will be able to answer.
When I have run into these problems I have been able to fix them by floundering around and managing to somehow fix things without understanding the root cause.
I've had permissions or user access issues with music files on the NAS a few times. Once happened when I bought a download that came in a ZIP or some other archive format. I copied the archive to a NAS folder and unzipped the archive there. The unzipped files had weird permissions and user account settings. I was unable to move those files or delete them using Windows. After that experience I now unzip archives on a local Windows drive before copying the files to the NAS. That avoids the problem. But I still don't understand the root cause for why that has happened.
A Windows utility that I've found useful is
Free Commander. It's freeware. It is able to scan folders on a NAS and find files or folders that are hidden or have other attributes. It's a very handy file management utility.