This is indeed a bug.
But even if the test code would work you'll have to remember that calling "Add" method won't do anything because sv was create using recursive fakes.
tip: in Isolator 5.2.0 Recursive Fakes is the default behavior.
Instead of using WillReturn use WillReturnCollectionValueOf that is intended for what you need:
[Test, Isolated]
public void SimpleViewTestOld()
{
SimpleView sv = Isolate.Fake.Instance<SimpleView>(Members.ReturnRecursiveFakes);
var instance = Isolate.Fake.Instance<MyBase>(Members.ReturnRecursiveFakes);
Isolate.WhenCalled(() => instance.FirstNumber).WillReturn(2.0);
Isolate.WhenCalled(() => sv.MyInt).WillReturnCollectionValuesOf(
new List<MyBase>
{
instance
});
foreach (var VARIABLE in sv.MyInt)
{
}
}