Status: Fixed. V1.40n-B has been reuploaded with this fix included, so redownload it if you already grabbed it.
Steps:
1. Assign a disarmer to a lemming.
2. Have the lemming disarm the trap.
3. Press any hotkey for backwards frame skipping (but do not press the level restart and replay button).
4. Watch the lemming after backwards framestepping - he will walk past the trap instead of disarming it. All other lemmings will walk past the trap safely.
5. However, if you press the hotkey to restart and replay the whole level, the lemming will be seen disabling the trap again.
Fixed. Since this was uncovered so soon after an update, and this is quite major, I reuploaded the recent update with this fix included.
Which I think also sets a new record for "time between a bug in NeoLemmix being reported and an update that fixes it being released". :P