Known issue when using Lightweight Solution Load - bimbam2103/space_engineers GitHub Wiki

Visual Studio allows a new feature where the solution is loaded in an alternative way, giving access to the project faster by loading projects in the background. This has a distinct effect on MDK: It cannot detect the fact that your project is an MDK project before the solution is completely loaded. So either disable Lightweight Solution Load for your scripting solutions, or wait until it is finished - which kinda makes lightweight solution load moot anyway.