Wix Setup Project breaks unrelated Migrations update-database call - entity-framework

Question

Running update-database from the Package Manager Console on an ASP.NET MVC Web Application fails after I add an unrelated Wix setup project to my solution.
Why is update-database accessing this unrelated project? Why does the presence of the wix project trigger a load of an assembly that cannot be found? How do I fix this?
This happens in a larger solution with web, desktop, installer projects, but after considerable head-scratching I reproduced this in isolation:
Setup a new stock-standard VS solution with an ASP.NET MVC Web Application, Entity Framework, enable migrations, add WebMatrix components, setup Role provider.
Add Configuration.Seed() method to call out to WebMatrix.WebData.WebSecurity.InitializeDatabaseConnection(..) and System.Web.Security.Roles.
Run update-database. It will work fine. Now add a Wix setup project (no need to configure) and it will fail at/before executing Seed() with
Could not load file or assembly 'Microsoft.Build.Framework, Version=15.1.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies. The system cannot find the file specified.
So, it seems having a Wix project in my solution triggers the load of an additional (not found) assembly when trying to call WebSecurity / WebMatrix methods through update-database from the Package Manager Console.
Workaround: Unload the Wix setup project and restart Visual Studio. There is no need to remove the project entirely.
Adding the missing dll to the GAC or a reference to the machine.config does not help. The dll is found but loading it errors out with
System.IO.FileLoadException: Loading this assembly would produce a different grant set from other instances. (Exception from HRESULT: 0x80131401)
Using FUSLOGVW.exe to log assembly load events didn't help. It basically shows that update-database can't find the v15.1 dll because it's not in the GAC and there is no app/host/system config file pointing at it.
Running the seed method from the running web application (enable automatic migration) works fine.
And yes, I am setting PMC "default project" to the web application, ie, point update-database at the web application.
Details:
Visual Studio Professional 2017 on Win 10 Pro fully patched.
Recently upgraded VS from 2015. No such problem under 2015.
Wix Toolset 3.11.0.1528
Package Manager Console 4.1.0.2427
System.Web.Mvc 5.2.3.0
Entity Framework 6.1.3
WebMatrix.Data.dll 3.0.0.0
WebMatrix.WebData.dll 3.0.0.0
Microsoft.Build.Framework.dll v15.1.0.0 lives in C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\MSBuild\15.0\Bin\ and Visual studio has a redirect to it in its app.config.
Snippets:
Configuration.cs:
class Configuration : DbMigrationsConfiguration<MyDbContextHere>
{
public Configuration() { AutomaticMigrationsEnabled = false; }
protected override void Seed(Stash context)
{
WebSecurity.InitializeDatabaseConnection("DefaultConnection", "UserProfiles", "UserId", "UserName", autoCreateTables: true);
if(!Roles.RoleExists("G1")) Roles.CreateRole("G1");
}
}
`
web.config:
`
<configuration>
<system.web>
<roleManager enabled="true" defaultProvider="SimpleRoleProvider">
<providers>
<clear />
<add name="SimpleRoleProvider" type="WebMatrix.WebData.SimpleRoleProvider, WebMatrix.WebData" />
</providers>
</roleManager>