Engine recompiles EVERYTHING on every launch?!

Hi!
Recently my UE 4.27 project started recompiling EVERYTHING on EVERY launch… 20.000 shaders. It’s just like when you open up a new tech demo downloaded from the marketplace.
It has never done this before. I use Sourcetree for version control since 2 years. I don’t know why this happens or how to fix it!
Any detectives out there?