Hello,
Version 15.0 beta 6 (15A5219j)
It doesn't save if you use isAutosaveEnabled: false. At first glance, I see what you described that the object in a list was changed, but if you terminate your app and open it again, you will see the date wasn't changed.
Of course, if you didn't use modelContext.save()
So the bug is here, I suppose.