incidentally, the game file I have isn't update=true, so I don't know if this was observed already.

Need to investigate. We either need to increase structural hp or even have a flag to always preserve the wreck.

it's a simple case of somebody removing the line from the ship's .xml
it only has to be returned to leave a wreck, no code work needed.
old .xml :

cyberDefenseLevel= "8"

explosionType= "&vtThermoExplosion2;"
leavesWreck= "100"

recent .xml :
cyberDefenseLevel= "8"

explosionType= "&vtThermoExplosion4;"

@shane: You're reading the XML correctly, but unfortunately, I pulled the rug out from under you. A few versions ago I made the engine auto-compute the "leavesWreck" parameter based on total ship mass. I was tired of constantly having to specify it.

For arks, worldships, and defenders, the chance of a wreck is set to 100% by default.

That's why I think it has to do with structural HP. Modern weapons are doing more damage than before, and I think the calculations for structural HP don't take that into account.

 From the sound of it, what happened was that it had a lot of explosive loot that a hit from a WMD weapon set off.

So maybe that is also why Salvager Nomads are blown to dust so easily in my last games. Does not happen when they have no loot, only when they are loaded.

I remember removing ammo, fuel, and other volatile loot (aside from special loot that happens to be volatile themselves) from my special quest ships to minimize the chance of the wreck exploding like a crate bomb due to some stray shot.

In 1.8 Beta 4 I've added more structural HP to wrecks. The calculation used to be proportional to the square root of the mass (0.5 power); now it is proportional to the 0.65 power of the mass. In practice, this increases the structural hp for large ships. E.g., Salvagers increased from 300 structural hp to 1200 structural hp. And Xenophobe ark increase from 4,200 to 24,000.

Structural hp are also used in the calculation to figure out if the wreck will explode, so this will help.