Please read what I posted previously. It's just a button to set or unset files read only. As soon as you close and reopen properties it will ALWAYS show as a solid box even if zero files in the folder are set read only. It's NOT an indicator.
You may use that option to set or unset all files in the folder read-only, but that's not the root of the issue. Something is interfering, causing Office to see the file as in use. It could be your AV software. It could be OneDrive. It could be EFS.
See this article: