You will find that any plastic will distort if not trash itself as soon as you bake it when using Fimo unless it is only being baked for a very short time.
I left some plastic bases imbeded in fimo when I did a base for some Warmaster figures as part of a GD entry and the bases curled up away from the fimo during the baking so I am guessing that it may have a similar effect with the figure.