How to reproduce:
1. Download the attached project(see notes for instructions)
2. Open the project
3. Open the files via an external script editor(reproduced with Rider):
Assets/LunaRuntime/Apollo/Events/EventValues.cs
Assets/LunaRuntime/Luna/Game/LunaImGuiWindows.cs
4. Add a comment to each of them and save, wait ~10 seconds without switching focus to Unity
5. After ~10 seconds switch focus to Unity
6. If after recompilation the errors do not appear - retry from step 3

Expected result: no errors are thrown
Actual result: "Reference to type 'Vector4' claims it is defined in 'System.Numerics', but it could not be found" errors are thrown

Reproducible with: 2019.4.8f1, 2020.1.2f1
Could not test with: 2018.4.26f1(project incompatible), 2020.2.0a20(Project starts with numerous errors, but the issue is not reproducible)

Notes:
2020.1.2f1 version of Unity displays errors after loading, but the issue was reproduced after upgrading from 2019.4.8f1
2020.2.0a20 version of Unity displays errors after loading(and requests API updating after launching), but the issue was not reproduced after upgrading from 2019.4.8f1