I second my sibling comment -- just submit a bug. I think you are wrong that they would ignore it just because their context is somewhat different. Linux is full of differences like this.
But also: Isn't one of the main benefits of NixOS that you can fairly painlessly get and try different versions of things? Just see if you can reproduce the bug in the same version of upstream. If so, file the bug with kde, if not, file the bug with NixOS because it's presumably caused by one of the patches.
This concern is a bit overblown here. There are 23 KDE patches at the moment, across multiple packages. kwin itself has 5, plasma-desktop has 4. Each one has an extremely limited scope - mostly one line changes related to paths rather than visible functionality.
If you have a specific issue under nixos, it's going to be fairly easy to see if it's very unlikely to be caused by custom patches.
From my experience, reproducibility is king. If you can give a formula to reproduce a bug, even if it is obscure, it makes it so much easier to track down.
That can be up to and including a downloadable VM image that shows the issue.
But also: Isn't one of the main benefits of NixOS that you can fairly painlessly get and try different versions of things? Just see if you can reproduce the bug in the same version of upstream. If so, file the bug with kde, if not, file the bug with NixOS because it's presumably caused by one of the patches.