Im my case also no ctrl-z, the spidertron was already placed in the save long time ago. I can provide the save pre-conversion (sadly autosave ticked after the disappear happened so i had nothing on between), but as i said, i didn't manage to reproduce it.

I just verified and i was sitting in the ...

definitely not, there were no rockets equipped

I know this will be very difficult to reproduce, so I'm wondering if it happens more often or i hit a very rare bug

upgraded a save from 1.1.107 to 2.0.10, played for about 4 minutes and spidertron I was in completely disappeared without a trace. Nothing in log at the time it happened.

when I ...