The code I posted was a stripped down example of a typical AAA-style test.
Please note that I'm NOT faking UsedInDummy itself, I'm faking Dummy. I needed a fake 'Dummy' (NOT a UsedInDummy!!), so using the AAA syntax I set it up like:
Dummy sut = Isolate.Fake.Instance<Dummy>();
It is kind of unexpected that, given Dummy has a property of type UsedInDummy, the static properties of the latter would not be initialized... This caused the test to fail because some code down the path used the static member from UsedInDummy.
I still consider this to be a (severe!) bug...