I'm not sure if I agree with you Dex. I don't know if what brian_s says is true, but if the movement order (or algorithm for movement) is indeed that way round, then it may well be an explanation for what's going on.
If the game really does first check whether you can perform the move independent of the platform then it is checking whether you can kill the wraithwing without moving the platform - in this particular case this indeed *would* be impossible.
The goblin can't kill a wraithwing suspended over pit or water. The only way the goblin could kill the wraithwing is if the platform was already under the wraithwing, but that's not happened yet.
If this analysis is correct, then I'm not sure this can be counted as a bug.
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