If that's the rule, it must be followed, but IMO it's terrible for the original author.
Let me tell you how I would feel if instead of being the updater I was the original author.
For instance all my (very few, so far) customs are full band but do not have Pro Guitar. Someone decides to add Pro Guitar to one of my customs e.g. Cosmic Girl but cannot ask me first because I was away for long. I come back and see one of the following:
1) There is a second Cosmic Girl entry in the database with Pro Guitar. It doesn't use my chart. That's totally ok. I might be a bit sad that now everyone downloads the more complete version but that's only fair if it's better.
2) There is a second Cosmic Girl entry in the database with Pro Guitar, and it uses my charts for the other instruments. This will piss me off to know end! Even if there is a line crediting me for 90% of the job, the new entry is under someone else's name. Many people will not remember the credits, only the uploader. This to me is a gross misappropriation of work without consent.
3) Someone has posted a link in a comment under my Cosmic Girl entry, to download a version that's identical plus Pro Guitar. It would not bother me at all, because people are still coming to my page to download it.
Besides all this, since I would certainly consent to someone adding Pro Guitar to my customs, in case 3) I would offer the upcycler to actually have their version replace mine, and add very clear credits for their addition. In case 2) I would sue them for copyright infringement.
No that's not true
of course I would not sue anyone. But I would definitely feel I had been robbed, and I would probably not share my future charting work again.