Simplest repro is to play a level that has a saved replay where the first skill assignment is to a right-facing lemming. Click button on toolbar to turn on left directional-select. Now load the replay. Observe that the first skill assignment getting played back fails to actually take hold, because the game seems to be applying the current left-directional-select mode even on replayed skill assignments, and therefore apparently refuses to make that replayed assignment to the lemming facing right.
In addition to save replays, this bug also affects replayed assignments from rewinds as well.
Fixed in commit c9dbb5a.