MSBuild not copying all DLL references

Question

I have a C# .NET 4.5 solution like this:

Project A.dll
-> Third Party.dll

Project B.dll
-> Project A.dll

When I build Project B, Third Party.dll isn't copied to the bin folder. However, it works if I add a reference to Third Party.dll like:

Project B.dll
-> Project A.dll
-> Third Party.dll

It also works if I do something like this inside Project B:

var t = typeof(ThirdParty.SomeClass);

I don't remember having this issue in the past. Any ideas?

I'm using Ninject v3.2 and Ninject.Extensions.Conventions v3.2. Is it possible that Ninject does dynamic assembly loading behind the scenes, and the compiler is running optimizations that strip out unused libraries?