Just got another Nebula as salvage from a debris field after completely destroying a pirate bounty fleet. The fleet definitely had no Nebulas in it originally, neither did mine. Unfortunately I do not have a save from before I destroyed the fleet or the debris field was generated, but I can send the save from after the debris field was already generated if that helps.
I'm currently on version 0.9a-RC9, still receiving the Galatia stipend, and not on commission with anyone - if any of that helps.
Spoiler
(https://i.imgur.com/6gVrgXl.png)