vtk/vtk!5682 launched tests off a really old branch. That caused build failures since some cmake flags or build state needed a clean build to work correctly. Since we only do incremental builds on MRs (unless Do: test --clean is explicitly requested), all subsequent MR tests failed due to similar errors until a master build got scheduled and cleaned the build dirs.
Moral of the story: please don’t test branches that are too old, and if unavoidable, use Do: test --merged.