Compile error when switching between branches from new format back to branch with old format

I'm trying to upgrade some libraries in a big solution, it's a step by step process, one lib at a time, so I'm using a separate branch along the way. When switching back to main branch to do some unrelated work I noticed nothing would compile anymore.

Note I'm not sure if msbuild is the correct place to file this issue, the error comes from the nuget.target.
Please let me know where to file otherwise and I'll open an issue there.

### Steps to reproduce
- Create a new VS project (Class Library, .NET Framework) 4.5.2
- Make sure it compiles, commit changes to git
- create new branch "convert" (or whatever)
- delete the assemblyinfo.cs
- Unload project, replace all with
```

netstandard2.0

```
- Reload, make sure it compiles
- Commit to this branch
- switch branches back to master (or whatever the branch was)
- Try to compile
- Get angry because it says
```1>C:\Program Files (x86)\Microsoft Visual Studio\2017\Professional\MSBuild\Microsoft\NuGet\15.0\Microsoft.NuGet.targets(186,5): error : Your project is not referencing the ".NETFramework,Version=v4.5.2" framework. Add a reference to ".NETFramework,Version=v4.5.2" in the "frameworks" section of your project.json, and then re-run NuGet restore.```
- I thought VS cached something, but even deleting the .vs folder completely and reoping the solution still leaves me unable to compile my project.
- Note that switching to the netstandard branch again compiles fine

OS info:

Win 10, VS 15.6.2 Professional