From this closed NL bug. (https://www.lemmingsforums.net/index.php?topic=6247.msg98589#msg98589)
@Crane - can you remember exactly how you managed to set this up in your screenshots? I'm having difficulty replicating exactly what you have there.
However, I have noticed that this happens, and I wonder if it's one and the same bug:
(https://i.imgur.com/yx2nVAV.png) (https://i.imgur.com/0Qqxbon.png)
Left: Fencer seems to be able to continue past the steel | Right: Fencer has found steel and stopped working, but walks forwards rather than turning, and the "chink" sound isn't cued
EDIT: Fixed in commit 5a57a8c