Unexpected failures on Hadrian

On my wip/derived-refactor branch, I recently did a Hadrian build and run of the testsuite, with the devel2 flavour. I got a bunch of spurious test case failures. 😭

Unexpected failures:
   /run/user/1000/ghctest-n6mvumis/test   spaces/testsuite/tests/ghci.debugger/scripts/break006.run           break006 [bad stderr] (ghci)
   /run/user/1000/ghctest-n6mvumis/test   spaces/testsuite/tests/ghci.debugger/scripts/break006.run           break006 [bad stderr] (ghci-ext)
   /run/user/1000/ghctest-n6mvumis/test   spaces/testsuite/tests/ghci/scripts/ghci011.run                     ghci011 [bad stdout] (ghci)
   /run/user/1000/ghctest-n6mvumis/test   spaces/testsuite/tests/perf/compiler/MultiLayerModulesDefsGhci.run  MultiLayerModulesDefsGhci [bad exit code (99): test timeout] (ghci)
   /run/user/1000/ghctest-n6mvumis/test   spaces/testsuite/tests/perf/compiler/MultiLayerModulesNoCode.run    MultiLayerModulesNoCode [bad exit code (99): test timeout] (ghci)
   /run/user/1000/ghctest-n6mvumis/test   spaces/testsuite/tests/ghci/should_run/T15369.run                   T15369 [bad stdout] (ghci)
   /run/user/1000/ghctest-n6mvumis/test   spaces/testsuite/tests/ado/T16135.run                               T16135 [exit code non-0] (normal)
   /run/user/1000/ghctest-n6mvumis/test   spaces/testsuite/tests/ghci/T16525b/T16525b.run                     T16525b [bad stdout] (ghci)
   /run/user/1000/ghctest-n6mvumis/test   spaces/testsuite/tests/ghci/scripts/T7627.run                       T7627 [bad stdout] (ghci)
   /run/user/1000/ghctest-n6mvumis/test   spaces/testsuite/tests/indexed-types/should_fail/T8227.run          T8227 [bad stderr] (normal)
   /run/user/1000/ghctest-n6mvumis/test   spaces/testsuite/tests/stranal/should_compile/T9208.run             T9208 [exit code non-0] (optasm)
   /run/user/1000/ghctest-n6mvumis/test   spaces/testsuite/tests/typecheck/should_fail/TcCoercibleFail.run    TcCoercibleFail [stderr mismatch] (normal)

By contrast, here is the output from make fast running the testsuite at the same settings:

Unexpected failures:
   ghci/scripts/ghci011.run             ghci011 [bad stdout] (ghci)
   rts/T18623/T18623.run                T18623 [bad exit code (1)] (normal)
   ghci/scripts/T7627.run               T7627 [bad stdout] (ghci)
   indexed-types/should_fail/T8227.run  T8227 [bad stderr] (normal)

Three of the failures are genuine.

Why are the rest happening? Once again, I'm back solidly in make-land.