We've got an issue with Word Online in our SharePoint Server 2016 (on-prem) environment.
If you open a document in a doc library in Word Online (i.e. in the browser) and start editing it, your editing session is retained even after closing the document. Then, if you try to do anything with the document properties like change the title or check it out or delete it, you get an error. Deleting it, for example, results in "the file is currently checked out or locked for editing by another user".
For extra fun, if you then reopen the doc in your browser, you get ANOTHER editing session logged that is retained. You can see these piled up on top of each other when you open the doc in the Word client and review the "Share" panel.
This seems like a problem with Word Online, since the issue persists even after clearing browser cache and using different browsers.