Here's the GitHub issue:

https://github.com/mrc-ide/covid-sim/issues/116

The discussion isn't terribly clear to me. Is this non-determinism in which random number gets used where, like if you seed an RNG once and then use it from multiple threads that get scheduled in varying order? That wouldn't be good, but the model would still be performing as intended, just with no way to make it deterministic for testing. Or is it some other kind of non-determinism?

I'm not sure anyone knows, which indeed isn't totally comforting. I'm not sure the "bug uncertainty" is any bigger than all the existing known uncertainty about the assumptions that go in to his (or any such) model, though.