Yes thats true, you have to edit the Tags before you rename it. ogg is a valid opus container, but most of known Tools can't edit the Tags inside the container.
MP3Tag can also edit the container, or renamed opus files
https://www.mp3tag.de/en/index.html Mp3tag v2.96 — This version adds support for Opus in files with *.ogg file extension, improvements when copying cover-art via the clipboard and fixes many issues and includes other changes.
See the Release Notes for more details.
2019-04-18
But we are still at Version 3.xx
Sorry, I have no better choice and if sindre will change that, who knows. As far as I know the (for me) best maintained Airsonic will have the same problem.