Changing a directory's name gave the message: 'The document "Blah blah blah.txt” could not be saved. You don’t have permission.' Why did I get this error?

I have a .txt file in a directory. Whilst the .txt file was open, I changed the directory's name. When I tried to save the .txt file, TextEdit / MacOS said I didn't have permission. If I go back into the directory and double-click on the .txt file, I can save with no problems. Finder confirms that I have read & write permission for both the directory and the .txt file. How can I fix this? (A while ago, I could change directory names no problem but these days MacOS whinges.)