(1.6.0) Undoing actions with Ctrl-Z resets the part model of FTL series fuel tanks - why?

[MattJL]

This should be an interesting one...

So I'm currently working on updating my ships from 1.4, and after screwing up and reaching for the trusty Ctrl and Z keys, I found that the part model on the FTL series fuel tanks reset!  This appears to happen even when starting from scratch:

I threw together a simple ship.

I put something on the top so I can undo a step.

The tanks revert back to their original models.

 

What on Earth could be causing this?  They're persistent enough that even when I load a saved ship, the models aren't retained.  Have I contaminated my save by bringing over old .craft files?

 

EDIT:  Just discovered that this happens even when reloading a save.

Edited December 21, 2018 by MattJL