An exported sim has an SIMO (outfit) and a SIME (sim) file, plus a couple of pictures. Seems simple...
And when making a human looking plumbot, you remove the single plumbot SIMO file and replace it with a human sim SIMO file with the same identity as the removed plumbot file... all well and good. Now you have a glitch ridden human looking plumbot that loses it's face when it travels to the future.
But I've noticed other differences... the human looking Sim has the traits and likes of the sim you stole the outfit from. Which kinda means those traits are stored in the SIMO file?! That doesn't seem right. Also, I made a plumbot out of an alien SIMO file.
Doesn't matter which of the four methods I know of that you use to create a plumbdroid, they just don't travel, the game always tries to make them a regular sim on arrival.
Plumbots should only have two outfits (Everyday 1 & Singed 1), so when you get the missing face check with MC to see what it's using (MC - Basic - Outfit), chances are it's using a bugged Everyday 2, change it back to 1 and remove the bugged outfit with MC.
Not sure if it's in the SIMO or not but traits and trait chips are saved when you save the bot to the bin, which means a fresh built bot using that .sim file has around 14 traits/chips affecting it, despite it's chip slots being empty.