See Armani's slider bug report topic, which also mentions this shimmier issue:
https://www.lemmingsforums.net/index.php?topic=5525.0As I mention there, it is
technically an expected outcome of other physics details (specifically, in this case, the shimmier doesn't quite end up far enough below the level bottom boundary to actually die), but it's definitely a corner case that needs fixing.
Workaround in existing levels is to add water or fire along the bottom, or just make the bottom-touching terrain not Shimmier friendly.
This
will be fixed in 12.12.