This is something I've seen off and on ever since pirates were introduced to the game.  Ocassionly, and randomly, a Ship Graveyard will NOT be defended by pirates.  I was never sure if this was Working as Intended or not, so I never sent in a ticket on the issue.  Also, because it was so random that it didn't always survive a restart/reload it made it difficult to show the bug in action.

But after looking at the code and after playing with a mod that also had its own custom Ship Graveyard encounters, I'm fairly sure this is indeed a bug.

So I have just sent in a ticket on this (#JIB-999-22474) where I have SIX ships capable of surveying anomalies parked next to six Ship Graveyards.  So far, for me at least, one of them always does not trigger a Ship Graveyard encounter no matter how many times I reload or restart (though they aren't always the same Ship Graveyard).

Hopefully this will help track down this random bug.

Turns out this isn't a bug at all, as I missed the "Encounter Chance" tag.

Whoops.  Egg's on my face now.   Please disregard this and the ticket I sent, SD (already posted on the ticket "thread" to disregard it).