If you have an issue of a file being in a Read-only state and no one else has the file open, verify that the Preview Pane is not turned on. The Preview Pane can hold the file open which will result in read only file since there are two copies opened.
The message below is what a user was getting.
To verify that the Preview Pane is turned off, Open File Explorer > Select View at the top > select Preview Pane to turn on/off.
Preview Pane turned on
Preview Pane turned off