I'm pretty sure this is a bug. I've tried this twice with two different plumbots and got the same result.
I sent my sim into the future and had him make a plumbot, I customized it the way I liked it, and then later sent my sim back into the present. Upon returning to the present, my plumbot was completely different from what I had made it in the future.
Here's what my plumbot looked like when I first made it and customized it in the future

And here's what my plumbot looked like when it came out of the time portal when I sent him back with my sim into the present
