I think that was the original idea but it may have been taken away by the update. I wondered why Android so suddenly reversed its decision to block use of an SdCard but the device manufacturers would prefer us not to do that. Also, some Service Providers might tweak Android to do what they tell it to.
That kind of interference could explain this rather bizarre behaviour.
That said, as webworkings points out, if you have it set to Portable Storage you have no chance but the App wouldn't have installed there in the first place.
The mystery deepens - are you sure you installed an App and not just copied it to the card?