Evaluated a CAF (0x7c8a18) that was GC'd!

I'm getting this on executables built with -ticky with a recent HEAD

bash$ ./gamteb 5000 
gamteb: internal error: Evaluated a CAF (0x7c8a18) that was GC'd!
    (GHC version 8.11.0.20200402 for x86_64_unknown_linux)
    Please report this as a GHC bug:  https://www.haskell.org/ghc/reportabug
Aborted (core dumped)

That's extremely alarming. This is in nofib/real/gamteb.

I've also seen it happen on nofib/spectral/mate.

Is anyone else seeing this? Can I do anything to provide more info?