Hmm. I can not confirm any problem with the last animation frame on importing a correct X model. But there are some X exporters that produce incorrect models, which also seems the case with yours.

When I view your model with the latest DX9 mesh viewer of April 2006, it looks totally garbled. Thus I suppose that there's something wrong with animation data of the model.

When I use the old DirectX mesh viewer of October 2004, the model looks correct, but the animation seems to be the same as in MED. It's hard to tell because the old mesh viewer does not allow to stop at a certain frame. But it seems that the MED import behaves similar as the working mesh viewer.

Which program were you using for creating and viewing that model?