How would they even know to ask upstream for support, rather than the fork? If you use package bar and have a problem, you would have to do a lot of digging to even find original progject foo and ask them for help (which wouldn't even make sense since it's a separate thing)

I am not using NixOS but in general every package manager links to the project homepage let’s say (or name of the project which for example in a GitHub fork is clearly different than the original) which in this case would be the fork.

So for example if this will be the case with a package I use I will report the bug to the fork because the package manager showed me their link.

I will not search for name-like projects or the original one and report there if a fork was distributed to me.

This is why I am confused why a fork is not solving the burden of possible bug reports. Also the fork assumes its own maintenance this is the purpose of the fork.