Pirate defended anomalies don't always stay defend by pirates

Just an observation I've seen in the game lately, and I don't know if it is a bug or WaI.  Ship Graveyards which are defended by pirates initially might, dozens of turns later, no longer have pirates defending them.  Initally I wasn't sure if these were anomalies I hadn't accounted for, but I have confirmed that, occasionally at least, they will disappear.

Maybe they get spooked and go somewhere else.

Anyway, I don't know what the actual mechanic here is, how many turns need to pass by, or if it is a percentage chance.  But I thought I would throw this out to the rest of the community to see if they've seen something similar.  If this is Working as Intended and not a bug, that means that Ship Graveyards that are being ignored due to not having enough firepower to take them on can have a chance to be revisited before the armed survey ships come into play.