I believe the collision isn't "checked" again.
A workaround can be to set the Sprite.Y to self.Y -1, which will be hardly visible, and make your character move through the platform as intended.
I believe it is due to some fix that was brought to the platform/jumpthru behaviors, reported in last version and fixed in r140.
At some point, you require such workarounds as Ashley already mentioned the platform behavior was becoming a very complex one.
Should help you going until Ash comes back from his time off.