Ok, since no work is against the task yet then I am assuming that there is no hold on the start date (I am also assuming no constraint set on the start date). You still have all three resources on the task, right? Is this task Fixed Work or Fixed Duration? I am assuming Fixed Work since changing a resource changed the duration, is that right?
When you start adding multiple resources (even if one is an asset) the tool doesn’t know that one resource is more important and the key one for this task. Therefore, the tool is working as designed – when you move the availability of one of the three resources, the system moves the duration to accommodate the two resources that are available. I don’t think that the problem is that you have an asset as a resource, I think it might be that this is a more important resource than your other ones in your mind but not in the system’s configuration.
As I think about this, I am wondering if the solution is actually NOT the resource. I think that what you are really saying is that “if this simulator is not available, then this task can not be done”. If that is true, I would put a task calendar on that task. And, you can have both the Simulator Resource and the task on the same calendar. That way the task will only be able to be worked on when the asset is available.