Well, the existing Lemmix program (ie. without any of your recent changes) does flag terrain piece #9 as having invalid terrain ID of 50. The real question though is, what happens in DOS's LemEdit or Customized Lemmings programs? It's interesting that the level screenshot on camanis.net did not depict a broken terrain piece there, unlike in Lemmix.
I'll have to dig up those old programs to test, maybe later this weekend.