When the platform reaches 512 the player (or pawn) simply falls through the floor of the platform.

At there limitations with this entity? ( oh yeh... using 0.75B)
you'd be surprised, one wrong figure entered in and book 9ft bounding box. i've pulled my hair out trying figue out a problem and it was because of a similar thing,in my test level, the ceiling was many times higher than 512, it was more like 2000 texels high
Yes that could be it. In an old book for the Genesis3d-Editor (which is almost the same as the RF-Editor nowadays) they wrote that it would be good to have 'smoth-frames' to help the editor in creating smoth animations.another thing to think of is check your moving platform check where the frames are is there skip forcing the animation to upbruptly jump making the plaform teleport above the player, try delete out the frames and re-add them see if that helps