This is no bug. You need to use c_move to update the position of the collision hull or wait until the end of the frame, then the hull will be updated automatically. You can set f.i. IGNORE_PASSABLE | IGNORE_MODELS | IGNORE_WORLD as the c_move mode, but when you set IGNORE_SPRITES, too (so all possible objects get ignored), then I think the hull won't get updated.

@jcl: I would like to see a function to update the collision hull manually so we don't have to use c_move for this purpose. A moving platform usually does not need collision detection, but its hull has to be updated before the end of the frame so the position/ height of the objects does not lag one frame behind.