ok, I did some experimenting and thinking, and I concluded that the error is not in the way MSI interacts with exposure, but that it is set too low. MSI should be a parameter "how bright are the fireflies permitted to be". But setting it to 10 classifies most of your scene as fireflies (because it is relatively hard scene, lots of indirect lighting). The problem does not happen with rectangular lights, because they make the scene easy (lots of direct lighting). Point the rectangular lights into the ceiling and you will get the same result.
Anyways, because the scene renders fine with high MSI, I think the best solution is to set the default MSI higher