I don't have the link here, but essentially yes, nextcloud can update it's own app code in it's image because you have to mount the code to your own filesystem. This means that between docker images you can have a mismatch of the code that you have stored and the code that the image is expecting, which frequently causes mismatches for me. This is an antipattern. The code should be stored in the image, not as a volume mount. There should never be a mismatch of code in a docker image - that's the whole point. The configuration could be out of date sure, or if there's a data file that's needed, that's expected. The actual running code thought, that should never be on a mountable volume.

Next time you update the image you will probably be greeted with a "Nextcloud needs to update". That should not exist. You already pulled the image, that should be everything you need to do. The caveats are extensions, kind of a grey area in my book, but I know it's not a clean pattern with those either. (The best one I've seen lets you pin the extension version with environment variables or a config file, and then once again you are in control of when they update, and no running code is stored outside of the image.)