I completed the five missions in the demo, then bought the full game on Steam. I completed a couple more missions in the full game (for a total of seven), then noticed that the Simulacrum still says "Complete the first five story missions to unlock".
I'm aware I can just click the "Unlock anyway" button. Just reporting the bug.
A save file for a mission isn't really relevant, so I'm attaching my GameSettings.xml file.
The Unlock was implemented late, probably after the first cut demo was released. So you might have to just replay Mission 5.
I noticed the same bug. After playing mission 7, Simulacrum was still locked.
I then quit the game, restarted, and completed mission 8. Then Simulacrum was unlocked. I suspect it unlocked when I quit and then restarted the game, but I did not check then, I only checked after completing mission 8.