See attached level. This was observed in v12.12.5, but it should still be present in the 12.13 RC build as well. I first noticed this while playing and solving 400 metres 16 from DavidLems and this took me by complete surprise as well. Currently, when using a teleporter, Lemmings can turn into zombies before they appear or pop out of a receiver if a zombie happens to walk into the receiver trigger in the meantime. IMO, they should only be able to be turned into zombies after they leave the receiver, not anytime before that.
Unrelated to this, but I checked with Lemmings using an exit, and it seems that they are completely immune to zombies once they start exiting, which makes sense because Lemmings are savable, zombies are not. So, IMO teleporting Lemmings should be immune to being turned into a zombie until they come out of the receiver.