Raiders will steal animus stones ("take what they can, and leave"), but the item's description says it's normally ignored by them. I've had this happen on two different raids, and it happens when I spawn a raid, then spawn an animus stone. They just walk up and take it (if it's uninstalled), or uninstall it and then take it.
Worng description or interpretion i think.
Raider's won't attack/destroy the stones like they would do for other installed colonist structures.
But they would take anything that is free to pickup, and uninstalled stones count toward that too.
I think it's not a bug, thanks for reporting though.