Referencing additional assemblies, mcs.rsp not working

I have an assembly that I’ve loaded into my project as a plugin. It relies on three additional assemblies that are not standard for a .Net 4.x profile. So I’ve created the following mcs.rsp file in the root of my asset folder.

-r:System.Drawing.dll

-r:System.Data.dll

-r:System.Data.DataSetExtensions.dll

I’ve tried everything I can think of, and the Unity compiler can’t find the System.Data.DataSetExtensions assembly.

FileNotFoundException: Could not load file or assembly ‘System.Data.DataSetExtensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089’ or one of its dependencies.

Any thoughts on what I might be doing wrong? And, is there a reason Unity can’t automatically find these kinds of standard assemblies and include them automatically?

It must be a Unity bug.

My project works in 2017.4.9f1 and 2018.1.0f2.
It fails in 2018.2.2f1 and 2018.2.3.f1.

(sigh)