OK... uh... I'm not sure why but now it works. I deleted these lines from my csproj:
<MonoGameContentReference Include="**\*.mgcb" />
and it started working. Then I thought, "oh cool that's the reason. Let me put those lines back and see it break again." And I put them back and it still works. So yeah, that's fun.
For reference, the project doesn't have any content files, those are in the shared projects.