Upgrading from net46 to net461 breaks NuGet package reference

I have a project targeting net46, that references a NuGet package. This works fine.

This package references a .dll (Package.Reference) that is included in its lib folder.

When the project moves to target net461, this reference no longer works. The package is still obtained, but it gives error NU1101 on Package.Reference. Why does a minor version change impact how a package is being loaded, and how can this be fixed?

The package in question, in case it's helpful.