Curiously enough, when I play the level in DOSBox I get the "lite" version too. I'm almost certain both DOSBox and Lemmix are reading from the same levelpack file, since I don't have multiple copies of it. IIRC my version is from Abandonia.
Ooh okay, I see now. <lightbulb> In that case it's smelling like a Lemmix bug. Perhaps the terrain data in question in the level file are marked in such a way that's not the usual "valid" combinations of flags, and Lemmix interprets the combination one way while Lemmings does it differently. (I'm discounting the possibility that it's a DOSBox bug since it's too much of a coincidence that only the decorative terrain are different, but if someone can check the level in real DOS, please do so.)
It would be interesting for someone to check what the level looks like in LemEdit. Not that LemEdit's rendering is guaranteed to be 100% compatible with Lemming's, but it's an interesting thing to compare. Also of interest would be if someone try cgalemmi.exe to see how the level renders in the CGA version.
(I would do it myself right now, but it looks like the copy on the computer I'm on at the moment is missing all these things I need to check.)
With this new information in mind, I think we'd have to say that only solutions that work on the "no decorative terrain" version counts. There are no two versions of the level; it now looks like it's just one version, but rendered differently in Lemmix vs PC Lemmings.