These pieces have somehow ended up with references to the fallback piece
actually written into the level file. These copies will not be possible to salvage for the most part - although in the second level, it looks like only the background has been set to the fallback one, so it might be recoverable by simply removing the background with a text editor, then loading the level and manually re-adding it.
So the bug you are directly seeing, is a result of bad data - with that being said, what I suspect has happened here is that the levels were cleansed by a copy of NL, that was missing styles that the levels used, therefore the default:fallback reference got written in their place. This shouldn't happen - NL should either refuse to cleanse levels that reference pieces that don't exist at all (as opposed to aliased, which it should continue to hardwrite the new name for), or keep the existing reference names - so in that sense, while not being able to load these files is expected, the fact that these files
exist in the first place (in their broken state, I mean) exposes a bug. I'm going to open a new topic for that bug. EDIT:
https://www.lemmingsforums.net/index.php?topic=5748.0In the meantime - try to hunt down the pre-cleanse copies of these levels, if you still don't have them, and see if you can play them as-is in NL. You might just need to download some missing styles - if you can download some missing styles and thus get the pre-cleanse copies working
in NL (not the editor), then once you
do have those styles present, you should be able to cleanse the levels again and this time the post-cleanse output should work. If not, then it is most likely that the levels reference pieces that don't exist anymore - possibly culled, possibly renamed.