Yes, that sounds wrong because it sort of contradicts itself with this version of stoppa, which occurs at a point in time and does not result in an ongoing state – stoppa when the police stops you refers to the act of pulling you over. It may lead to your becoming other things as an ongoing state, like häktad 'detained' or 'remanded'. So you can bli stoppad av polisen and then you may vara häktad or arresterad 'arrested' for some time afterwards.
I just thought of this very unlikely event: imagine a robot that was stopped by the police. If that robot were to tell you about what happened, would it say 'Jag blev stoppat av polisen'? (stoppat istället för stoppad). I know it's a stretch, but looking at the future, this could well be a sentence that could start making sense... :)