After rebuilding my steam rocket into a petroleum rocket, I launched it and saved the game.
After loading the game today, half of my rocket is displayed there still, but the real rocket is on mission!!
Mouseover is saying vacuum as you can see in the screenshot, os it's a graphical glitch.
But possibly also a gameplay issue, because I had my dupes filling and emptying excess oxylite (to the second module in the screenshot) in a permanent loop until removing the access ramp.
Steps to Reproduce
Build steam rocket (1 command, 6 research, 1 steam), replace lowest 3 modules by petroleum/liquid fuel / oxylite tank, start rocket, save game, load game.
This issue has not been confirmed by a developer yet.