I want my library to work with a range of versions of a NuGet package with breaking changes in API between the changes. I haven\'t investigated it furt
This is not a complete answer but I noticed on your GitHub issue page that you are referencing both .NET Standard and .NET Framework libraries in your project. This is known to NOT work correctly.
Quoting .NET Standard team's announcement,
.. Another symptom is warnings at build time regarding assembly versions ..
which may be what you are running into.