The change from a single recorded piece of output to multiple happens just as the library I'm testing is dropped in to the model.
I can't post it, but it contains such things not seen before as a discrete time integrator, and matrix operations like reshape or multiply, and selector blocks.
What is confusing me is why every single output has the same number of recorded pieces of data outputted as the number of outputs in the model. Signals that simply go in and out (with nothing happening to them) change from a single value going in to multiple (but similar) values coming out. The last value seems to be the one I want coming out, so I'd assume that's the same for all of them.