There doesn't seem to be a clear or simple way to get the physics behavior to behave properly with the platform behavior (or other behaviors). I covered this problem mostly in this topic:
Anyway, as I was saying before, in the event editor (under the "Physics" tab), there should be an option to enable collisions with an object. Currently, there only seems to be an option to disable collisions with an object.
I thought there was an "enable collisions with an object" option before, but I don't see it (at least not in the recent version of Construct). Since it doesn't seem to exist, I would like to request for it to be added as a feature.
Edit: Either that, or some other way to make two behaviors work together without problems, platform and physics especially.