From the IRC logs:
<mobius1>I just discovered you can platform while building and cause the lemming to immediately turn around; is this intended?
<Nepster>This is a consequence of
http://www.lemmingsforums.net/index.php?topic=2850.0<Nepster>There is no final decision yet whether to keep the change or go back to the old behavior or try something else instead.
<Nepster>Currently I am waiting to see whether players like the change or not.
<namida42>yeah, definitely not possible on the old version. i don't think this should be possible, but i'm also not sure how one would go about fixing this without either (a) un-fixing the miner corner issue or (b) introducing a special case for assigning platformers to a builder
<SimonN>(b) *for assigning platformers while you are inside terrain, which won't happen for walkers, only for builders
<IchoTolot>wait platforming a builder causes him to turn that's not good in my opinion
<IchoTolot>yes I am with Simon here (b)
<IchoTolot>I should probably post the log in the topic
<SimonN>except that I don't want to make the check particular to builder. I should have called this (c)
I think this should be hotfixed as turning lems while platforming on builders seems fishy.