Take the following code :

        ChangeToken.OnChange(() => _fileProvider.Watch(@".\test.txt"), FileChanged);

        private void FileChanged()
        {
            var file = _fileProvider.GetFileInfo(@".\test.txt");
            _fileExists = file != null && file.Exists;
        }

Tried with a PhysicalFileProvider.
If I call the FileChanged() method directly, the file provider is functioning properly and providing information on the file.
If I change the file, the change is not watched and the ChangeToken does not trigger.

If I update the code by removing the trailing .\, the ChangeToken is also functioning properly :

        ChangeToken.OnChange(() => _fileProvider.Watch("test.txt"), FileChanged);

        private void FileChanged()
        {
            var file = _fileProvider.GetFileInfo("test.txt");
            _fileExists = file != null && file.Exists;
        }

The GetFileInfo do not consider the trailing .\ as invalid characters, but the Watch does.